From snoop + -er.
snooper (plural snoopers)
- A person who snoops.
1938, Norman Lindsay, Age of Consent, 1st Australian edition, Sydney, N.S.W.: Ure Smith, published 1962, →OCLC, page 87:Bradly was embarrassed, detected in the character of a snooper. But he had to come on, short of bolting back in his tracks.