Green’s Dictionary of Slang

posh n.4

[posh adj. (1), i.e. the drug’s image]

(drugs) cocaine.

[UK]I. Welsh Filth 77: Anything other than posh is a waste ay time for that cunt.
[UK]C. Brookmyre Be My Enemy 98: No stranger to posh or whizz when he needed an eighteen- [...] hour session at the computer.
[UK]I. Welsh Decent Ride 36: Ah picks up some mair posh fae Rehab Connor n droaps it oaf tae Monny in leith.