Wiktionary
hang by a thread
vb. (context idiomatic English) To be in danger, calling for precise caution; to be in a precarious situation.
WordNet
hang by a thread
v. depend on a small thing or be at risk; "His life now hangs by a thread" [syn: hand by a hair]
Usage examples of "hang by a thread".
He did not mention that the bakery-owner would be away for only four weeks, and that after that period Miller's life would hang by a thread.