if you can't say something nice, you're not trying hard enough.

tongue tied.
stammering. stuttering. stupified.
don't know what to say.
have nothing to say.
can't think what to say.
have a nice day.

struck dumb.
cat's got your tongue.
befuddled. bewildered.
bedazzled. befiltered.
can't find the words.
at a loss.
don't disturb.

speechless. mute.
buttoned up. resolute.
hushed. shushed. on the DL.
on the QT. mum's the word.
bloody hell.