Pet form of John, also used as an independent given name from the 16th century onwards. In the United States it is occasionally also used as a girl's name. Famous bearers include the American country singer Johnny Cash (1932–2003) and the film actor Johnny Depp (b. 1963).
Possibly an altered spelling of German Pose (or Poser 2), a habitational name from Posa or Poserna, south of Merseburg, or a topographic name from Middle Low German pōse ‘lump’, ‘pile’ (from which the place names may also derive).