Warning of danger

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"significant, indicative," 1890, from Greek semat- , comb. form of sema (genitive sematos ) "sign" (see semantic ) + -ic . Used especially in biology, in reference to "warning" colors, etc.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Sematic \Se*mat"ic\ (s[-e]*m[a^]t"[i^]k), a. [Gr. sh^ma, sh`matos, sign, token.] Significant; ominous; serving as a warning of danger; -- applied esp. to the warning colors or forms of certain animals.

Wiktionary Word definitions in Wiktionary
a. (context biology of the colouring of some insects etc English) Acting as a sign of danger.