gull, n. — Green’s Dictionary of Slang

gull n.

[SE gull, to dupe, to deceive; note gull, simpleton, dupe, despite inclusion in Grose (1785) is SE]

a trickster, a cheat.

1598		Jonson Every Man In his Humour IV ix: Why how now, signior Gull! are you turn’d filcher of late!
1600		Dekker Shoemakers’ Holiday IV iv: These two gulles will [...] take master Bridegroome, and mistris Bride napping.
1606		Return from Parnassus Pt II II v: How say you to the crafty gull.
1613		Beaumont & Fletcher Honest Man’s Fortune III iii: Didst thou never hear of one Montague, a prodigal gull, that lives about Paris?
c.1698		B.E. Dict. Canting Crew n.p.: Gull c. a Cheat.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1825		C.M. Westmacott Eng. Spy I 161: You’ll excuse me, sir, but as you are fresh, take care to avoid the gulls. [note] Gulls – knowing ones who are always on the look out for freshmen.

In compounds

gull-trap (n.)

(US) any establishment designed to fleece the innocent and gullible.

1850

Manchester Spy (NH) 7 Sept. n.p.: Dr Ingalls’ gul-trap [...] will be attended next week. We would advise the community to avoid this place as you would a serpent [...] at your feet.