I thought it meant either hostage taker(1), hat trick(2), or hat tip(3). It also seems that most people who use this abbreviation don't really know what is means.
If (one) then you are a bad guy.
If (two) then you just did three successful attempts of something, usually in sports.
If (three) then you just acknowledged someone, similar to the saluting of officers of higher ranks in the military.
There probably are more meanings to this H-T phenomenon but no seems to be the wiser.