I asked him what he meant by plunging.
'Oh,' he says, 'a plunger is a fellow who makes a great turnout, don't you know, and leaves cards at the best houses, and is sought by the mamas, and strolls in the Park very languid, and is just a hell of a swell generally. Sometimes they even condescend to soldier a little--when it doesn't interfere with their social life. Good-day, Mr Flashman.'
Flashman, George MacDonald Fraser (1969)