Usually spelled "cack-handed". There's a long tradition the the right hand was the "clean" hand used for feeding and the like whilst the left was used for cleaning up your, well, cack. If you use toilets in Egypt you'll find a little valve on the right hand side of the bowl that directs a spray upwards, you use your left to clean. No-one would ever touch food with their left (however well washed), it's incredibly dirty and bad manners. So, someone whose dominant hand is their left is "cack-handed", and since left-handers are often portrayed as clumsy, there's your explanation. This prejudice seems universal, to the Romans the right was "dexter", think dexterous, clever, able whilst the left was "sinister".