Green’s Dictionary of Slang

1. (UK Und.) to defraud one’s criminal confederates, to divide booty unfairly.

1811		Lex. Balatronicum n.p.: To Well. To divide unfairly. To conceal part. A cant phrase used by thieves, where one of the party conceals some of the booty, instead of dividing it fairly amongst his confederates.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1832		Satirist (London) 13 May 159/1: They are paid a weekly sum, generally about £2; [...] and the money given them to bonnet (that is, to play) with, is always stamped by Leaming with a private mark, to prevent any welling.
1859		Matsell Vocabulum.

2. to pocket; thus well it, to be well-off, to make a good income.

1828		‘A Flat Enlightened’ Life in the West I 159: The division being made, and the money ‘welled,’ Lord Hulse rang the bell.
1860		R. Nicholson Rogue’s Progress (1966) 27: For about every second sovereign she took from the floor she ‘welled’ in her shoes.
1864, 1867, 1870		Hotten Sl. Dict.

3. to conceal a proportion of one’s income or estate from one’s creditors.

1893

Illustrated Sporting and Dramatic News 22 July 766/1: Out of the salvage of my fortune – for something had been safely ‘welled’, you may be sure – I purchased a tricycle [OED].