To be not up to Dick was to be ill — not well; ill and wretched, in J. Redding Ware's definition from Passing English of the Victorian Era (1909).

Ware's label for it is one of his most charming: Common Respectable Life. Not the street, not the stage, not the thieves' kitchen — this was a phrase for decent households, the sort you might use to a neighbour over the fence.

It makes most sense as the negative of a phrase that was then current: up to Dick, meaning up to standard, as good as it ought to be. Green's Dictionary of Slang finds that in Punch in 1879, in one of its comic 'Arry pieces, written in a Cockney voice: Our party was quite up-to-Dick. From the 1880s Australian papers were using it of sausages, funerals and newspapers alike. Something up to Dick passed inspection. A person not up to Dick did not — they were off colour, below par, out of sorts.

As for who Dick was, the honest answer is that nobody seems to know. The phrase behaves like up to scratch or up to the mark, where the standard is named but never explained, and we have found no reliable source that settles it. Guesses are easy to come by; evidence is not.

It is a gentle, slightly comic way to say you feel rotten, which is probably why it suited respectable life. It admits to being unwell without going into detail.