I came across the word in an article about Bristol Zoo, which has set up an amphibian sanctuary to breed two endangered species. One of them is the golden mantella frog of Madagascar, which is a brilliant golden-orange. The colours are aposematic, referring to the bright markings or hues exhibited by some living creatures to warn predators that they are poisonous. (The frog cheats: it isn’t toxic but the colours fool its enemies into thinking it is. Some writers have restricted aposematic to such false warnings.)
Though this is common enough in the biological sciences, it’s not often encountered elsewhere. Here’s a rare example:
A gigantic bird of prey was descending on him, its claws outstretched. Its aposematic wings were spread wide, as wide as the field itself. Looking up in shock, Hungaman saw how fanciful the wings were, fretted at the edges, iridescent, bright as a butterfly’s wings and as gentle.
Aboard the Beatitude, by Brian W Aldiss, 2002.
The word is from classical Greek, based on sema, a sign, which also appears in polysemous, the coexistence of many possible meanings for a word or phrase, semantic, relating to meaning in language or logic, and semaphore, the method of communication that uses flags or other signs. The prefix apo- means “away, off, from”.