1766, from Mod.L. masturbationem, from L. masturbatus, pp. of masturbari. The long-standing speculation is that this is altered (probably by influence of turbare "to stir up") from *manstuprare, from manu, abl. of manus "hand" (see manual) + stuprare "defile" (oneself),
from stuprum "defilement, dishonor," related to stupere "to be stunned, stupefied" (see stupid). But perhaps the first element represents an unattested *mazdo- "penis." An earlier technical word for this was Onanism (q.v.).