Q From Gord Forsythe: When someone says, ‘He’s really got his work cut out for him,’ I take it to mean that he will have difficulty in successfully completing his work or task. But I would think that if he were a tailor, and had his work cut out for him, that would simplify his task.
A The origins of the phrase are somewhat obscure, as is so often the case with idioms.
The Oxford English Dictionary’s entry suggests it was first recorded in the sense in which we now understand it only around the middle of the nineteenth century. The first appearance in the sense of “to have (at least) as much as one can handle” recorded in the OED is in A Christmas Carol by Charles Dickens, which was published in 1843. I’ve found some examples from the previous century, but their meaning is ambiguous.
That’s because the expression goes back at least to the early 1600s in a related form, “to have all one’s work cut out”. As you suggest, it was borrowed from tailoring, but in that first figurative sense it meant to prepare or plan an activity, to get everything organised before starting work, as a good tailor would. It later went through a period in which it meant that someone else cut out your work for you, that is, gave you something to do.
The image behind the current sense is that of having some assiduous assistant cutting the cloth at such a rate that it’s a struggle to keep up.