Green’s Dictionary of Slang

a boon companion, a true friend, a trusted servant; often as trusty trout n.

1607		Middleton Michaelmas Term II iii: I perceive the trout will be a little troublesome ere he be catched – Boy!
1640		R. Brathwaite Ar’t Asleepe, Husband? 91: [T]his tractable Trout had no sooner heard than he became well perswaded: humbly beseeching his Friend to excuse his errour .
1664		C. Cotton Virgil Travestie (1765) Bk I 49: Madam (quoth he) your humble Trout.
1688		T. Shadwell Squire of Alsatia I i: Your humble trout, good noble squire.
1705–07		N. Ward Hudibras Redivivus II:6 8: Tho’ ’tis, by jolly Trouts, confest, / The cask that sounds the least, is best.
1839		(con. 1703) W.H. Ainsworth Jack Sheppard (1917) 25: Gentlemen, your most obsequious trout.