To Answer a Query

A cack-handed young lass enquired as to the origin of 'Cack-Handed.  Naturally I had absolutely no idea!  So when I came home from the museum this afternoon having a sleep, stuffing my fat face, and watching Scotland freezing in Norway I decided to investigate.
I didn't get far.
Not one person could give a clear lead as to the origin.  Many could define the word however:

The Oxford dictionaries say:
Definition of cack-handed in English
British informal
1: inept; clumsy: a great song ruined by cack-handed production
2: derogatory left-handed.

Urban Dict

1. cack handed

It means an awkward or inept way of doing something; originally meaning left handed, 

stemming from cultures that use their right hand to eat 

and their left hand to wipe their behind.

Dictionary Central

cack-handed

Definition: Slang

adjective:  clumsy, inept. 
The term originally meant left-handed, probably deriving from the idea of handling cack (excrement). Although the connection seems obvious, this expression is probably too old to be influenced by reports of the Muslim practice of eating with the right hand, wiping away excrement with the left. This pejorative adjective seems to be country dialect in origin; it is now fairly widespread and not particularly offensive.

Origin and History of “cack-handed”

Cack comes from a 15th-century dialect verb meaning ‘defecate’, which probably came from middle Dutch cacken. It goes back via Latin cacāre to an ultimate Indo-European base *kak-, from which a lot of other Indo-European languages get words connected with ‘excrement’. The connection with cack-handed is usually explained as being that clumsy people make a mess; on this view ‘left-handed’, which cack-handed also means, is a secondary sense derived from ‘clumsy’. It may be nearer the mark to place ‘left-handed’ first, however, bearing in mind the traditional role of the left hand in many cultures for wiping the anus.

Somewhere else:

Cachus was Old English for a privy, and both words come from Latin cacare, to defecate.

The general consensus, with absolutely no proof, is that it refers to the use of the right hand for eating and the left for er, the other end.  The derivation from the Latin into English and the use English folk made of such genteel terms indicates this may be correct.  

Interestingly the word 'right' is used in many European cultures for 'correct.'  

The Latin 'SINISTRA' meaning 'left,' gives us 'Sinister.'  
People therefore considered anything of the left evil.
Make of that what you will!

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Addendum:
A left hander writes-
You seem to have omitted lots of stuff about all cack-handers being geniuses... 

'Cack,' a diminutive of crack, meaning elite, special, esp. referring to military units...

hmmm...

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