not for a moment

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.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.