Hair doesn't weigh anything until after it's dead. amirite?

That doesn't make any sense. A hair follicle is alive and it has weight

I'm not high enough to understand this

Want to weigh your hair while it's living and breathing? Simple. Weigh your whole nude body first. Shave all the hair off your body and head then weigh yourself again. You'll weigh less and that difference will be what your once alive hair weighed.

It has mass therefore it has weight anywhere in a gravity field

Do you mean because is attached to the body?