1. A person who, as a result of frequenting clubs and other such locales, becomes devoid of dignity, civility, and class.
2. A person, who, on account of promiscuity, has become crass and lude.
Originates in Tolstoy's Anna Karenina book 7
wherein he uses the analogy that a slosher is someone like a hard-boiled egg which has been turned over too many times and has become cracked, and unappealing.