Congratulations. You're about to read an in-depth discussion of the world's dirtiest undergarment: John McClane's tank top. Y'all, this tank top goes through the ringer.
For reference, check out this shot of McClane at the beginning of the movie. His tank top is blissfully white—perfectly untarnished. And this is what it looks like by the end of the movie. It seems McClane has found some time to take a fully clothed mud bath during the Nakatomi siege. (N.B. Technically, by the end of the movie, the tank top is so far gone that McClane is actually shirtless, using the tank top as a bandage for his bloody feet.)
This poor, poor tank top's descent into dirtiness parallels John's arc in the movie. The more beleaguered our Mr. McClane gets, the grosser his tank top appears. As he shimmies through air ducts, hurls himself out of windows, and shoots bad guys, he's bound to get a stain or two or twenty. For every lick he takes, the tank top takes two.
The tank top similarly highlights McClane's vulnerability. When Gruber and his henchmen take over, John's busy washing up in the bathroom, utterly unprepared for cowboy cop mode. He's half dressed and barefoot. We wouldn't call that battle gear. The dirtier his tank top gets, the more vulnerable he is in the movie. And finally, he sheds that tank top in his most vulnerable moment of all—when he's hiding out in the bathroom, nursing his bloody feet and pouring his heart out to Al Powell.
P.S. Fun fact: A while back, Bruce Willis donated the tank top he wore in the movie (or, more likely, one of many) to the Smithsonian. Now that's some culture worth preserving.