1
general::
As a settled matter, finally, permanently, as in Once and for all, we're not hiring that organist again, or We've settled that question once and for all. This expression is in effect an abbreviation for "one time and for all time." [Late 1400s]
American Heritage Idioms
2
general::
Phrase(s): once and for all
finally; permanently. • Sue: I’m going to get this place organized once and for all! Alice: That’ll be the day! • "We need to get this straightened out once and for all," said Bob, for the fourth time today.
McGrawhill's American Idioms And Phrasal Verbs