Entry 1 sense Webster, 1913 Illegitimacy /ĭlˌ-ĭj-ĭt'-əm-əs-ē/ · Il·le·git·i·ma·cy · IPA /[ɪlɨˈd͡ʒɪɾɨməsi]/ 01 n. The state of being illegitimate. 1. The state of being illegitimate.