Green’s Dictionary of Slang

(UK Und.) a well-off knight or any rich gentleman.

1595		Shakespeare Romeo and Juliet II ii: Hist! Romeo, hist! O! for a falconer’s voice, To lure this tassel-gentle back again.
1608		Dekker Lanthorne and Candle-Light Ch. 5: The Tercell Gentle that comes to the Lure, is some knight, or some gentleman of quality.
c.1698		B.E. Dict. Canting Crew n.p.: Tercel-gentle c. a Knight or Gentleman of a good Estate; also any rich Man.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1820		W. Scott Abbot iv n.p.: I marvel what blood thou art – neither Englander nor Scot – fish nor flesh. Marry, out upon thee, foul kite, that would fain be a tercel-gentle [F&H].
1853		Globe (London) 23 Apr. 3/3: Different species of hawks were assigned to different ranks of individuals [...] the ger-falcon and its tercel to a king; the falcon-gentle and the tercel-gentle to a prince.