1
general::
Except for, were it not for. For example, But for the afternoon shower, it was a perfect day, or But for the children, they would have gotten a divorce long ago. [c. 1200]
American Heritage Idioms
2
general::
Phrase(s): but for someone or something
if it were not for someone or something. • But for the railing, I’d have fallen down the stairs. • But for the children, Mrs. Smith would have left her husband years ago.
McGrawhill's American Idioms And Phrasal Verbs