Derives from the use of the word 'riding' to refer to sexual intercourse. It is undecided as to whether the word can be used only if the subject of the verb was on top (the one doing the horsing).
verb: drinking excessively, doing drungs excesively, having sex very rough with no respect for the girl, beating someone down
ive been horsing beers all day, i horsed a fat line last night, i horsed her so good she wont walk straight for a week, i horsed this dude repeatedly until he was in convulsions.