Green’s Dictionary of Slang

pimple n.1

[ety. unknown; ? link to pimp n.]

a boon companion.

[UK]Congreve Way of the World IV ii: The sun’s a good pimple, an honest soaker; he has a cellar at your Antipodes.
[[UK]Wodehouse Right Ho, Jeeves 204: A simple, jolly, kindly young pimple she had always struck me as].