An affectionate word used by Firefly fans to refer to the character of Mal, and (more rarely) the actor Fillion with reference to the show.
Mal: Just be careful. We cheated Badger out of good money to buy that frippery, and you're supposed to make me look respectable.
Kaylee: Yes, sir, Captain Tight Pants
Example 2 -
Fan: There was a rumour that Captain Tight Pants himself would turn up at that convention last week!