1
general::
Rescued from a difficulty at the last moment, as in I couldn't put off explaining his absence any longer, but then Bill arrived and I was saved by the bell. This expression alludes to the bell rung at the end of a boxing round, which, if it rings before a knocked-down boxer has been counted out, lets him get up and continue fighting in the next round. Its figurative use dates from the mid-1900s.
American Heritage Idioms
2
general::
Phrase(s): saved by the bell
Cliché saved by the timely intervention of someone or something. (Alludes to a boxer who is saved from being counted out by the bell that ends a round.) • I was going to have to do my part, but someone knocked on the door and I didn’t have to do it. I was saved by the bell. • I wish I had been saved by the bell.
McGrawhill's American Idioms And Phrasal Verbs