Slacks and Underpants

Slacks noun - (usually in the plural) pants for casual wear.

Underpants and slacks are semantically related in bloomers topic. In some cases you can use "Underpants" instead a noun "Slacks".

Underpants noun - An undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin.

Slacks and underpants are semantically related in bloomers topic. You can use "Slacks" instead a noun "Underpants", if it concerns topics such as clothing for legs.