1
general::
Phrase(s): not for a moment
not at all; not even for a short amount of time; never. • I don’t want you to leave. Not for a moment! • I could not wish such a horrible punishment on anyone. Not for a moment!
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Never, not in the least, as in Not for a moment did I believe he was telling the truth. This expression employs moment in the sense of "the tiniest length of time," a usage dating from the mid-1300s.
American Heritage Idioms