1832, "to go around in a prying manner," Amer.Eng., "to appropriate," probably from Du.
snoepen "to pry," also "eat in secret, eat sweets, sneak," probably related to
snappen "to bite, snatch" (see
snap). Specific meaning "to pry into other people's business" is attested from 1921. The noun meaning "detective" is recorded from 1891.
Snoopy (adj.) first recorded 1895.