What's the "een" in Halloween?
1690s, "piece of music for one voice or instrument," from Italian solo, literally "alone," from Latin solus "alone" (see sole (adj.)). As an adjective in English from 1712, originally in the non-musical sense of "alone, unassisted;" in reference to aircraft flying from 1909. The verb is first attested 1858 in the musical sense, 1886 in a non-musical sense. Related: Soloed; soloing.