Not too bright
These are one-liners that you really don't want people to be using to describe you. I think it's nicer to use these though instead of saying that someone is just unintelligent. If they aren't intelligent though, they may not understand what you're saying about them. If they're the type to seek revenge that may just be to your benefit.
1. Not the brightest light in the harbor.
2. The light's on but no one's home.
3. Not the brightest bulb in the box.
4. A few screws short of a hardware store.
5. Not the sharpest knife in the drawer.
6. A few cards short of a deck.
7. A few fries short of a Happy Meal.
8. About as sharp as a marble.
9. Only has one oar in the water.
10. Smart as a bag of rocks.
11. A burger short of a combo meal.
12. The elevator doesn't go all the way to the top floor.
13. A few peas short of a casserole.
14. A few keys short of a keyboard.
15. Not the brightest bulb on the Christmas tree.
16. The gates are down and the lights are flashing, but the train isn't coming.
17. As smart as a stick.
18. Fell out of the stupid tree and hit every branch on the way down.
19. An intellect rivaled only by garden tools.
20. Has an IQ of room temperature.
21. Couldn't pour water out of a boot with instructions on the heel.
22. Not the brightest crayon in the box.
23. One twist short of a slinky.
24. Sharp as a sack of wet mice.
25. More numb than a frozen mukluk.
26. I ain't sharpest crayon in the box - Markie Sparkie Lover.
27. I ain't the sharpest tool in the shed - Markie Sparkie Lover.
28. They are depriving some village of its idiot.
29. A few threads short of a sweater.
30. A few sandwiches short of a picnic.
31. Driveway doesn't quite reach the road.
32. The battery is not fully charged.
33. About as sharp as a bowling ball.
34. Dumber than a bag of hammers.
35. A few bricks short of a full load.
36. A few clowns short of a circus.
37. A few beers short of a six-pack.
38. Dumber than a box of hair.
39. A few tacos short of a fiesta platter.
40. All foam no beer.
41. As smart as bait.
42. Chimney's clogged.
43. Doesn't have all his dogs on one leash.
44. Forgot to pay his brain bill.
45. The antenna doesn't pick up all the channels.
46. His belt doesn't go through all the loops.
47. Another brain would be lonely.
48. Missing a few buttons on his remote control.
49. No grain in the silo.
50. Proof that evolution CAN go in reverse.
51. Receiver is off the hook.
52. Has a leak in the skylight.
53. Too much yardage between the goal posts.
54. Dumb as a donkey.
55. Not all the soldiers are marching in line.
56. Dumber than paint.
57. Half a bubble off plumb.
58. A few fruit loops shy of a full bowl.
59. Donated his brain to science before he was done with it.
60. A few shades beyond blonde.
61. Has to take turns for the family brain-cell.
62. A few watts short of a light bulb.
63. Dump as a stump.
64. Running on 3 cylinders.
65. A few Bradys short of a bunch.
66. Has the parachute but is missing the ripcord.
67. Would lose a debate with a doorknob.
68. Has an IQ lower than plant life.
69. All telephone, no receiver.
70. One ski short of a snowmobile.
71. Wouldn't know if they were on foot or horseback.
72. The logs are ablaze but the chimney is clogged.
73. Eats soup with a fork.
74. The wheel is spinning but the hamster fell off (or is dead).
75. The cheese slid off the cracker.
76. The engine is running but nobody is behind the wheel.
77. A few fuses short of a full circuit.
78. A tire short of an eighteen wheeler.
79. Doesn't have all the chairs at the table.
80. Dumber than a bag of rocks.
81. A shining example of why you should avoid inbreeding.
82. A few pecans short of a fruitcake.
83. As fruity as a bag of Skittles™
84. Would argue with a signpost.
85. If you gave them a penny for thoughts, you'd get change.
86. Dumb as a salt shaker.
87. Has a mind like a steel trap: rusty and illegal in 37 states.
88. Knitting with only one needle.
89. The result of too much chlorine in the gene pool.
90. Not the brightest bulb in the Chandalier.
91. Not the quickest bunny in the forest.
92. The hard drive is spinning but the OS hasn't been installed.
93. Not exactly running on all thrusters.
94. A few toppings short of a Deluxe Pizza.
95. A few burgers short of a barbecue.
96. A few roos loose in the top paddock.
97. The wind is blowing but nothing is moving.
98. The umbrella is up but there's no rain.
99. A few colors short of a rainbow.
100. As bright as a lamp in Aladdin's cave.
101. The boat doesn't have all the oars in the water.
102. A few boats short of a fleet.
103. A monosynaptic cretin (Don't understand it? 'nuff said)
104. A few noodles short of a chow mein.
105. A few bristles short of a broom.
106. A few snags short of a barbie.
107. Doesn't know whether to scratch his watch or wind his butt.
108. Hasn't seen the ball since kickoff.
109. The relative IQ of a deck chair.
110. A poster child for birth control.
111. A few players short of a team.
112. Couldn't hit the floor if he fell on it.
113. A few sheep short of a flock.
114. If you stand close enough to them you can hear the sea.
115. They have an IQ lower than their shoe size.
116. A few gunmen short of a posse.
117. As sharp as a pound of wet liver.
118. Not the quickest horse in the stable.
119. Has a full six-pack, but lacks the plastic thingy that holds them all together.
120. Most people drink from the fountain of knowledge, they only gargled.
121. Not the fastest ship in the fleet.
122. Ten cents short of a dollar.
123. A few boxes short of a pallet.
124. A few grams short of a pound.
125. Shipped but not delivered.
126. A few springs short of a watch.
127. A few bits short of a byte.
128. Only has half a cord in the woodshed.
129. The elevator goes to the top but the doors don't open.
130. A cup and a saucer short of a place setting.
131. If their nose was on upside down they'd drown in the rain.
132. Three ice bricks shy of an igloo.
133. A few marshmallows short of a bowl of Lucky Charms.
134. A few bales short of a wagon load.
135. Couldn't find their way out of a paper bag.
136. A few needles short of a sewing kit.
137. One IQ point above brain death.
138. Any slower and he'd need to be watered once a week.
139. Playing hockey with a warped puck.
140. All booster, no payload.
141. Has some lug nuts rattling in the hubcaps.
142. Nice house, not much furniture.
143. Was hiding behind the door when they passed out brains.
Also: The wheel is turning but the hamster is dead.