Green’s Dictionary of Slang

a boon companion, a true friend.

c.1698		B.E. Dict. Canting Crew n.p.: Trusty-trout a sure Friend or Confident.
1705–07		N. Ward Hudibras Redivivus I:4 19: He’s a true Trout that scorns, Ads-fish, / To Porridge beg, and steal the Dish.
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 n.p.: Trusty Trout, a true friend.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.