To form a sac or pouch.
This word is used in biology to describe things that have a pouch or sac-like shape.
Examples:
Some animals can sacculate their stomachs to hold more food.
The plant sacculates its leaves to store water.
The frog's throat sacculates to hold its food.
I wish I could sacculate my pockets to hold more toys!
The superhero's suit can sacculate to hold all of his gadgets.
The balloon sacculates as you blow air into it.