Green’s Dictionary of Slang

1. a hand [fin n.¹ (1)].

1946		St Vincent Troubridge ‘Some Notes on Rhyming Argot’ in AS XXI:1 Feb. 47: strong and thin. Fin. (Origin uncertain, probably English.) Agreed, but rare compared to Lincoln’s Inn (one of the English legal Inns of Court in London).
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.

2. a £5 note [fin n.² (1)].

1960, 1975		Wentworth & Flexner DAS 648/2: Lincoln’s Inn – five dollars.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.

3. gin.

1972

Dodson & Saczek Dict. of Cockney Rhy. Sl.