unable to think of something to say.

Chris was lost for words at Kathy's incredible rudeness.

Usage notes: usually said because something has surprised you

• fly in the face of
• fly in the face of something
• fly in the ointment
• fly into a rage
• fly off the handle
• fly-on-the-wall
• fly on the wall
• fly the coop
• fly the flag
• fly the nest
• foam at the mouth
• foaming at the mouth
• fob off on
• fob off on someone
• fob something off on
• fob something off on someone
• foist on
• foist on someone
• foist something on
• foist something on someone
• follow in footsteps
• old guard
• old-guard
• old hat
• Old Nick