Refers to a garment worn next to the skin under outer clothing; can be a shirt-like undergarment. The term is somewhat formal or literary; in everyday speech, 下着 (したぎ) is more common.
He changed his underwear.
This undershirt feels good against the skin.
From Chinese 襯衣 (chènyī), meaning 'underwear' or 'lining garment'. The Japanese reading しんい is the on'yomi of the characters.