to become round and full of air, or to make something round and full of air
The sail bellied out in the wind and soon the boat was gathering speed.
belly out
To flare out in the wind, as of fabric. The sails bellied out in the afternoon breeze.
belly out
[for a sheet of fabric, such as a ship's sail] to fill out in the wind. The sails bellied out as the ship turned to catch the wind.When the sails bellied out, we began to move forward.