IPA: /ˈflæpləs/
KK: /ˈflæpləs/
Not having a flap or flaps; lacking a part that can move or cover an opening.
The flapless design of the envelope makes it easier to open and close.
Flapless is formed from "flap" (of uncertain origin, possibly imitative of the sound made) and "-less" (meaning without). The word "flapless" means without a flap or lacking a flap.
Think of something that is 'without' ('-less') a 'flap' — that's what flapless means.