1610s, "stating or introducing an inference" (of words such as because, then, therefore); 1630s, "inferential, arising from inference," from Late Latin illativus, from Latin illatus "brought in," used as past participle of inferre "to bring in, introduce" (see infer). Grammatical sense "case expressing motion into" is from 1890. As a noun from 1590s, "illative word." Related: Illation "action of inferring" (1530s).