unable to think of something to say.

If I was alone with her, I'd feel at a loss for words.

Usage notes: usually this happens because you are surprised

• a dirty trick
• a dirty weekend
• a dirty word
• a disaster area
• ad nauseam
• a dog and pony show
• a dog in the manger
• a dog's breakfast
• a dog's dinner
• a domino effect
• a done deal
• a Don Juan
• a double bind
• a double entendre
• a double whammy
• a doubting Thomas
• give a once-over
• give a piece of mind
• give a piece of your mind
• give a ring
• give a rocket
• give a rough time
• give a run for money
• give a run for their money
• give a run for your money