Do you know ghouls from goblins and ghosts?
1610s, from Late Latin soliloquium "a talking to oneself," from Latin solus "alone" (see sole (adj.)) + loqui "speak" (see locution). Also used in translation of Latin "Liber Soliloquiorum," a treatise by Augustine, who is said to have coined the word, on analogy of Greek monologia (see monologue). Related: Soliloquent.