for a very good reason; A spoken lead-in before a blunt, critical, or nosy remark, signaling the speaker is about to say it anyway.
Oxford60% of films
for a very good reason
Well, I used to date a pinball champ, not for nothing.
Editorial40% of films
A spoken lead-in before a blunt, critical, or nosy remark, signaling the speaker is about to say it anyway.
You know, Mookie, not for nothing,
Hey, not for nothing, dude, but this kid really hates his mother.
Definitions from the Oxford Advanced Learner's Dictionary, 9th edition