calzoncillos in English:

I was wearing a T-shirt and underpants.

My underpants are wet.

I want a pack of underpants.

He had a shirt on and underpants and I kept telling him not to go back into the house, but he wanted some trousers to wear.

Please take off all your clothes except your underpants and bra.

We're going to buy underwear. You're still not wearing underpants, are you?