Tees maar khan is a phrase I'm familiar with from my very childhood. 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.
The earliest and probably the only memory of the phrase for me is of the times I used to be proud of something and my father used to say, 'Tum khud ko bahut bade tees maar khan samajhte ho?' (You think you are some big tees maar khan?) which would mean I wasn't someone great who had done some great thing and there was nothing I should be actually proud of.
So even though I have never looked for the word in the dictionary, I guess I have conveyed you what Tees Maar Khan means.