means The garments worn next to the skin beneath your outer clothes — underpants, undershirts, and the like.

from A plain English compound, exactly what it says: "under" (the old Germanic word for beneath, kin to German "unter" and Latin "infra") plus "wear," from the verb "to wear," which once simply meant to clothe oneself. The pairing as a noun for undergarments is a relatively modern assembly, born of the polite need to name such things without naming them too closely.