1832, "to go around in a prying manner," American English, probably from Dutch snoepen "to pry," also "eat in secret, eat sweets, sneak," probably related to snappen "to bite, snatch" (see snap, v.). Specific meaning "to pry into other people's business" is attested from 1921. Related: Snooped; snooping.

1891, "act of snooping," from snoop, v.. Meaning "one who snoops" is from 1929; meaning "detective" is from 1942. snooper "one who pries or peeps" is from 1889.