Green’s Dictionary of Slang

a large denomination sterling note.

1857		in J. Archbold Magistrate’s Assistant Long-tailed finnips Large notes.
1895		Crowe Aus. Sl. Dict. 28: Finnuf, a five pound note; double finnufs, a ten-pound note; long-tailed finnufs, large notes.
1905		Exeter & Plymouth Gaz. 4 Feb. 5/6: £10 is a ‘double finnip,’ or a ‘long-tailed finnip’.