We almost sold the car, but the buyer backed out yesterday.
The sale was practically finished, but the buyer suddenly changed their mind and canceled the deal.
Verb: back out
Vibe: Behavior and social impression
To withdraw from a promise, agreement, or plan after you already said yes.
He promised to help us move, but he backed out at the last minute.
You picture someone agreeing to a handshake deal, but then slowly taking a step backward away from the group and slipping out the door while everyone else gets to work.
Real-life examples
Don't back out now—we already bought your ticket!
The speaker is urging a friend not to break their promise to go on a trip, especially since money has already been spent.
Synonyms
Contrast & nuance
'Back out' focuses on breaking a specific commitment or contract. 'Bail out' or 'flake' are more casual slang for flaking on social plans, while 'chicken out' specifically means quitting because you got too scared.