Origin and history of stool pigeon

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.

[pretense or evasive story to avoid doing something] 1851, slang, earlier stall-off (1812), from the earlier sense of "thief's assistant" (1590s, Greene, "Conny Catching," also staller), especially a pick-pocket's assistant who diverts the attention of the victim and assists in the escape, from a variant of stale "bird used as a decoy to lure other birds" (early 15c.), from Anglo-French estale "decoy, pigeon used to lure a hawk" (13c., compare stool pigeon). The etymological sense is "standstill." It is from Old French estal "place, stand, stall," from Frankish *stal- "position," which is ultimately from Germanic and cognate with Old English steall (see stall (n.1)).

Compare Old English stælhran "decoy reindeer," German stellvogel "decoy bird." The figurative sense of "deception, means of allurement" is recorded by 1520s. Also compare 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]