[While three children sneaking up a set of staircases to the third floor under the Invisibility Cloak] They didn't meet anyone else until they reached the staircase up to the third floor. Peeves [=a fairy] was bobbing halfway up, loosening the carpet so that people would trip.
"Who's there?" he said suddenly as they climbed toward him. He narrowed his wicked black eyes. "Know you're there, even if I can't see you. Are you ghoulie or ghostie or wee student beastie?"
He rose up in the air and floated there, squinting at them.
"Should call Filch, I should, if something's a-creeping around unseen."
Harry had a sudden idea.
"Peeves," he said, in a hoarse whisper, "the Bloody Baron has his own reasons for being invisible."
Peeves almost fell out of the air in shock. He caught himself in time and hovered about a foot off the stairs. (Harry Potter and the Sorcerer's Stone)
Is it –– caught himself –– an idiom: it has the meaning ‘to hold oneself back abruptly from saying’ in Collins?