Literally Tees Maar Khan means someone who has killed thirty men. In many stories, mostly those of Sheikh Chilli, Tees Maar Khan is a man who pretends to be a tiger of a man claiming he has killed thirty men, though it doesn't seem so. Hence, when you talk of the actual meaning of the phrase, it's used for someone who says he has done something really big, something worth being proud of, though it may not actually be so.

30 machchhar maarne wale ko...........lol.