IPA: /ˈkoʊt.ləs/
KK: /ˈkoʊtləs/
Not wearing a coat or lacking a coat.
On a warm day, he felt comfortable being coatless.
Coatless → It is formed from "coat" (from Old French *cote*, meaning a garment or outer covering) and "-less" (meaning without). The word "coatless" means without a coat or lacking a coat.
Think of someone who is 'without' ('-less') their 'garment' ('coat'). This helps you remember that 'coatless' means lacking a coat.