IPA: /ˈkɜːrtənləs/
KK: /ˈkɜrtənləs/
Not having any curtains present.
The room felt more open and airy because it was curtainless.
Curtainless → It is formed from "curtain" (from Old French *cortine*, meaning a hanging cloth) and "-less" (meaning without). The word "curtainless" means without a curtain or lacking a curtain.
Think of a space that is 'without' ('-less') a 'hanging cloth' ('curtain') — that's what curtainless means.