Green’s Dictionary of Slang

lay n.4

[? SE lay, to place; i.e. that which is laid on the table, counter etc.]

1. a piece, a portion, e.g. a lay of pannum, a piece of bread.

[UK]D. Haggart Life 49: We had a weighty lay of them that same evening.
[UK]Hotten Sl. Dict.
[UK]Sl. Dict.
[UK]W.J. Barry Up and Down 61: He offered me a ‘tenth lay,’ i.e. a tenth of the entire profits.

2. goods.

[UK]D. Haggart Autobiog. 7: My want of knowledge of the flash kanes, where I might fence my snib’d lays.