Not entirely clear. Possibly from Old Latin *insquom, compare īnsece and īnseque (“say!”, imperative). From Proto-Indo-European *h₁en-sékʷ-t, the *h₁en-prefixed form of the aorist stem of Proto-Indo-European *sekʷ- (“to say”) that has been thematicized in both Latin and Ancient Greek ἐνέπω (enépō, “to tell”).