d. colloq. A customer, a client; (sometimes) spec. a prostitute's client.
1965 Sunday Times 24 Oct. (Colour Suppl.) 66/3 There is plenty of irrational judgement about..but like all free-market operators, the traders have to concentrate it on people—on each other and on the ‘punters’ (dealer buyers).
First known use in 1965 according to the OED.
