Entry 1 sense Webster, 1913 Onomastic /(ŏn`ō̇*măs"tĭk)/ · On·o·mas·tic · IPA /ˌɑː.noʊˈmæs.tɪk/ 01 a. Applied to a signature when the body of the instrument is in another's handwriting. 1. Applied to a signature when the body of the instrument is in another's handwriting.(Law)