9 Q Without U Words for Words With Friends
1650s, "one who peeps," agent noun from peep (v.1). Slang meaning "eye" is c.1700. From 1590s as "young chicken" and 1857 as "tree frog" (American English), both from peep (v.2).
A private detective; pi, private dick: And don't bother to call your house peeper (1940+)