From Longman Dictionary of Contemporary Englishbe lost for wordsbe lost for wordsSAYSURPRISEDto be unable to say anything because you are very surprised, upset etc For once in her life, she was lost for words. → lost
Examples from the Corpus
be lost for words• For once in her life, she was lost for words, and uncertain of her argumentative ground.• He was lost for words at the time, and had to apologise and thank the donors later in private. 