related discussion: Who would have thunk?
related discussion: thunk
related discussion: thunk

NOAD:
thunk¹ - (n. & v.) informal term for THUD.
thunk² - informal or humorous past and past participle of THINK: who would've thunk it?

Webster's 3rd unabridged:
¹thunk - dial past of THINK
²thunk - a flat hollow sound {heard the dull thunk of wood against wood —Brian Harwin}

OED:
thunk, sb.¹ Joc. var. THINK sb.
1922 Joyce Ulysses 503 Have a good old thunk.
thunk, sb.² (int. or adv.) [Onomatopoeic.] A sound of an impact, whether dull or plangent. Also int. as an adv.
1952 B. Harwin ... He heard the dull thunk of wood against wood and felt the planking jar over his head. 1958 'W. Henry' ... Presently the sodden 'thunk!' of an ax blade caving in barrel staves echoed wetly. 1968 W. Garner ... The door said thunk in a well-bred whisper. 1970 M. Chisholm ... The bullet tore through the canvas of the cover and went thunk into a barrel. 1971 A. Ross ... I heard the triple thunk of the undercarriage locks. 1979 Herald (Melbourne) ... The familiar 'thunk, zing, ding' of a pinball machine.
thunk, v. intr. To make a thunk; to fall or land with a thunk.
1963 S. Plath ... With shovels and picks they crawled through the attic trapdoor and soon great masses of snow were thunking from the roof into the yard. 1972 Daily Tel. (Colour Suppl.) ... A quoit which thunks into the clay just to the left of the hob, leaning towards it, bevel downwards, is called a pot. 1976 New Yorker ... Last night, I slept nine hours, rain thunking on the tent.
thunk, dial and joc. pa. t. and pa. pple. of THINK v.² Cf. THUNK sb.¹
1876 C. C. Robinson Dial Mid.-Yorks. ... Think .. (Thuongk) The last form is less employed participially than in the past, in which tense it is of constant occurrence. 1887 Lantern (New Orleans) ... Who'd a thunk it? 1909 N. Duncan ... Leastwise, he thunk so, admittin' 'twas open t' argument. 1939 Joyce Finnegans Wake 504 I then tuk my taken-place lying down, I thunk I told you. 1967 T. Savage ... Phil had most excellent use of the hides after all. Who'd a thunk it!

Another in the wildly popular 'dumpfes Geräusch' series ... (-: