OED spells it with a hyphen - http://www.askoxford.com/concise_oed/cackhand...

I've also heard the phrase as a big girl's blouse, but there's no reason why it couldn't be used without big.
http://www.worldwidewords.org/qa/qa-big3.htm

Both are common phrases in the UK.

Both correctly heard and spelt, and both common sayings.

Could it be "backhanded" ?

I know "cackhanded", m-w.com also has it:

http://www.merriam-webster.com/dictionary/cac...

I've never heard of the "girl's blouse", but it seems to be British. The "right" just makes it stronger ...

http://www.worldwidewords.org/qa/qa-big3.htm

And another confirmation from me: I know both cack-handed and (big/right) girl's blouse.
In fact, LEO knows cack-handed too: Dictionary: cack*

If I am correctly informed, this indeed stems from the "childish word for poo" (well, "poo" is in itself a childish word for poo, isn't it? :o)), or rather from the fact that one used the left hand (the "bad" hand, the hand with which right-handers are more clumsy) to wipe it off. Compare to the German linkisch.

This association is nonsense, of course,says left-hander

It never occured to me to check leo or other dicts., I was that convinced I'd misheard. Thanks.

(Yes, I suppose poo was "doppelt-gemoppelt" Dragon.)

Actually the word cack is an old word (i think gaelic) for the word poo so the word poo comes from cack.

And now I feel I'm misreading -- what do you mean Londoner? They are two different words for the same thing, right? Not that one developed out of the other?

The root meaning of cack-handed is left-handed; it's nowt to do with poo. My Dad sometimes used to mock me for being "cackhanded" when he saw me using things in a left-handed way, as I occasionally do even though I'm right-handed normally.

Cack-handed means left-handed, but the word "cack-handed" is connected to poo. Not sure if it is as Dragon described, or just like saying your left hand is shit.

cack-handed adj, colloq 1 clumsy; awkward. 2 left-handed. cack-handedness noun.
ETYMOLOGY: 19c: from dialect cack excrement.http://www.chambersharrap.co.uk/chambers/feat...

cack noun 
[origin Old English cac- in cachus (hus house) rel. to cack verb]
1. Excrement, dung; filth OE
2. fig. Rubbisch; nonsense M20

cack verb
[Origin Middle Low German, Middle Dutch cacken (Dutch kakken) from Latin cacare]
1. verb intrans. Defecate. LME
2. verb trans. Void as excrement. L15

cack-handed adjective
[Origin from CACK noun + HAND noun + ED.]
Left-handed; clumsy


So at least according to the SOED, the term cack-handed does derive from cack, which in turn relates to excrement.

So what is the level of vulgarity of "cack-handed"?

Not vulgar imho. Probably most people who use it don't even realize where it comes from.

Agree that it is not considered vulgar.
The Wikipedia site saying that cack-handed refers to wiping your bum with your left hand quotes Michael Quinion, who is usually quite reliable, and it's a plausible idea, though a little actual proof would be nice (he doesn't provide any). http://www.worldwidewords.org/qa/qa-cac1.htm

Caution:
Although most might not consider cackhanded to be vulgar, and cack certainly is vulgar, its use to label a left-handed person was certainly offensive to me as a left-handed child and was usually meant as such in my experience.
"Girl's blouse" is intended to be offensive in its use, although I've not been on the receiving end of this one.

Re #17: Well, I grew up in the English Midlands, am in my thirties and only became aware of any possible connection between the phrase "cack-handed" and left-handedness in the last couple of years. In my family, we only ever used it as a synonym for clumsiness involving the hands. (Dragon seems to have had similar experience to mine - see her post #10 in the thread linked by Marianne in #12: related discussion: kerr-handed - linkshändig (nach dem König Kerrs) - #10 )

I also agree that "cack-handed" is not considered vulgar - despite it containing the word "cack".

Coming from the UK Midlands [though it may not be exclusive to here] there is the alternative form "caggy-handed" which may be used to avoid saying the word "cack" but in this instance only.

I remember years ago hearing someone say (in BE) "He's a real big girls blouse"- I'd never heard it before but got the general drift pretty quickly. The statement was made about someone, not to their face; it's not vulgar, but definitely disparaging. It expresses quite a lot of disdain.