Green’s Dictionary of Slang

frogskin n.1

also frog
[such notes are/were green; Franklyn, Dict. of Rhyming Slang (1961), suggests rhy. sl. frogskin = SE sovereign]

1. [20C+] (US black/campus/Und., also alligator skin, fish-skin, froggyskin) a banknote, $1 bill.

2. [1920s] (US Und.) a counterfeit dollar bill.

3. [1920s–60s] (Aus.) a £1 note.