Please note spelling of underWEAR (not underWARE). Underwear is more like a collective noun - it refers to all kinds of undergarments - bras, knickers, pants, boxer shorts, vests etc. That is why we say "I'm going to buy some underwear" - we are being unspecific. If I go to buy underwear, I might come home with 2 bras, 3 pairs of knickers and one vest. I have still bought "underwear". It's uncountable.