From Longman Dictionary of Contemporary EnglishI can’t tell youI can’t tell youa)SECRETused to say that you cannot tell someone something because it is a secret‘Where are you taking me?’ ‘I can’t tell you; it would spoil the surprise.’b)DESCRIBEused to say that you cannot express your feelings or describe something properlyI can’t tell you how/why/what etcI can’t tell you how worried I’ve been. →tell
Examples from the Corpus
I can’t tell you• "So is Maggie really pregnant?" "I can't tell you."• I can't tell you how grateful I am for your help.