The shirt is red and it offers peep shows for a quarter. Xander wears it in "Out of Mind, Out of Sight" (1.11) and then Oz does in "Anne" (3.01). This is the shirt's biography in seven self-contained stories.
1. Purchased. [G] Willow drags Xander to the flea market; even the most amends-y guy in the world deserves something for himself.
2. Soiled. [NC-17] Larry drags Xander to a party and Oz soils the shirt.
3. Loaned. [NC-17] Xander drags Oz home and loans him the shirt. Things happen in between.
4. Wiped. [NC-17] Oz builds a cage and Xander procrastinates.
5. Darkened. [NC-17] Oz drags himself back to Sunnydale, Xander drives an ice-cream truck, and some chances just expire.
6. Torn. [R] It's Xander's turn to run away from Sunnydale, Oz is an unexpected homebody, and they both need to remember how to make peace.
7. Mended. [NC-17] They don't have to be cowards.
Thanks to Spuzz for the banner.