The ability for an often-times too small t-shirt to be used exclusively as an undershirt, mostly worn under polos. This term refers mostly to preppy teenage males but it can apply to many different male groups.
Walker: Man... I was brushing my hair for the third time this morning and I noticed that this T-shirt is way too small for me now.
Trey: Don't worry, its still totally undershirtable. I've got like a whole closet full of them.