c. 1200, aslepe, o slæpe, "in or into a state of slumber," from Old English on slæpe (see a- (1) + sleep (n.)). The parallel form on sleep continued until c. 1550. In religious literature sometimes euphemistic or figurative for "dead" (late 13c.). Meaning "inattentive, off guard" is from mid-14c. Of limbs, "numb and having a prickly feeling through stoppage of circulation," from late 14c.