to make a mistake or do something badly [= mess something ↔ up]:
He fluffed his shot and missed the goal.
She fluffed her lines in the first scene.
2 also fluff something ↔ up/out
to make something soft become larger by shaking it:
She fluffed up the pillows for me.
3 also fluff something ↔ up/out
if a bird fluffs its feathers, it raises them and makes itself look bigger