stool pigeon n.

"pretense or evasive story to avoid doing something," 1812, from earlier sense "thief's assistant" (1590s, also staller), from a variant of stale "bird used as a decoy to lure other birds" (mid-15c.), from Anglo-French estale "decoy, pigeon used to lure a hawk" (13c., compare stool pigeon), literally "standstill," from Old French estal "place, stand, stall," from Frankish *stal- "position," ultimately from Germanic and cognate with Old English steall (see stall (n.1)). Compare Old English stælhran "decoy reindeer," German stellvogel "decoy bird." Figurative sense of "deception, means of allurement" is first recorded 1520s. Also see stall (v.2).

The stallers up are gratified with such part of the gains acquired as the liberality of the knuckling gentlemen may prompt them to bestow. [J.H. Vaux, "Flash Dictionary," 1812]

late 14c., pijoun, "a dove, a young dove" (early 13c. as a surname), from Old French pijon, pigeon "young dove" (13c.), probably from Vulgar Latin *pibionem, dissimilation from Late Latin pipionem (nominative pipio) "squab, young chirping bird" (3c.), from pipire "to peep, chirp," a word of imitative origin. As an English word it replaced culver (Old English culufre, from Vulgar Latin *columbra, from Latin columbula) and native dove (n.). 

The meaning "one easily duped, a simpleton to be swindled" is from 1590s (compare gull (n.2)). Pigeon-hearted (1620s) and pigeon-livered (c. 1600) are "timid, easily frightened." A pigeon-pair (by 1800) are twins of the opposite sex (or family consisting of a boy and a girl only), so called because pigeons lay two eggs, normally hatching a male and a female.