Definition of "peeper"

Who's there? peepers, intelligencers, eavesdroppers?

1604 (first performance), Tho[mas] Dekker, Iohn Webster [i.e., John Webster], VVest-vvard Hoe. […], London: […] [William Jaggard], and to be sold by Iohn Hodgets […], published 1607, (please specify the page)

If listeners seldom hear good of themselves, it is also true that peepers sometimes see more than they like; and Betty, the cook, as she reached the landing, glancing askance with ominous curiosity, beheld a spectacle, the sight of which nearly bereft her of her senses.

1863, Sheridan Le Fanu, The House by the Churchyard

So you go to see this peeper, this Marlowe. That was your mistake.

1944, William Faulkner, Leigh Brackett, Jules Furthman, The Big Sleep (screenplay)