The Jargon File (version 4.4.7, 29 Dec 2003):

YHBT
 //

    [Usenet: very common] Abbreviation: You Have Been Trolled (see troll,
    sense 1). Especially used in ?YHBT. YHL. HAND.?, which is widely understood
    to expand to ?You Have Been Trolled. You Have Lost. Have A Nice Day?. You
    are quite likely to see this if you respond incautiously to a
    flame-provoking post that was obviously floated as sucker bait.