From Longman Dictionary of Contemporary Englishfor certainfor certainCERTAINLY/DEFINITELYwithout doubt SYN for sureknow/say (something) for certain I know for certain it’s in here somewhere.that’s/one thing’s for certain One thing’s for certain, he won’t be back. → certain
Examples from the Corpus
know/say (something) for certain• I didn't really know this for certain. 