c.1600, from L.L. soliloquium "a talking to oneself," from L. solus "alone" + loqui "speak." First used in translation of L. "Liber Soliloquiorum," a treatise by Augustine, who is said to have coined the word, on analogy of Gk. monologia (see
monologue). Verb soliloquize is recorded from 1759.