(Noun). A style of capri-like or long pants made of a loose, light, airy material mainly worn while lounging around in your apartment, or at a friend's, but can also be worn out for late-night trips.
Origin: Episode of "Friends" entitled 'The One With Rachel's Sister". Phoebe refers to Jill's "don't-you-want-to-rent-me-this-apartment? pants" as "apartment pants".
I want to change into apartment pants before Tiffany and Sara come over.