Dated term, within the BDSM community, for those who whip themselves, or are into such practices. Useful term, though out of favour among many who practise it.
Originally used to denote those in religious orders, that did the self flagellation ritual.
More recently used among Psychologists, in denoting those patients who are into BDSM routines, most notably that of being whipped, either by a partner, or by their own devices.
Flag (pronounced as fladge) is another method of spelling.
Shortened version of the word 'flagellation'.