Green’s Dictionary of Slang

Bush, the n.¹

In phrases

at the Bush (also at Staines) [the Bush Inn at Staines, ? a popular refuge for London debtors]

in financial difficulties.

1812

Vaux Vocab. of the Flash Lang. in McLachlan (1964) 269: staines a man who is in pecuniary distress is said to be at Staines, or at the Bush, alluding to the Bush inn at that town.