Refers to clothing worn over other clothes, such as coats, jackets, or robes. Somewhat formal or literary; in everyday speech, 上着 (うわぎ) or コート are more common.
It was cold, so I put on an outer garment before going out.
Because it was cold, I put on an outer garment.
As soon as I entered the room, I took off my outer garment.
コート refers specifically to a coat or overcoat, whereas 外衣 is a broader, more formal term covering any outer garment.
下着 means underwear worn next to the skin, the opposite of 外衣, which is worn as the outermost layer.
arrival at ... · counter for suits of clothing · nth place (in a race) · counter for moves (in go)
Compound of 外 (outside) and 衣 (clothing). The reading がいい is the on'yomi (Sino-Japanese) reading of the characters.