1. Definitely or positively
   "it's going to be a good day for sure";
   - surely, certainly, sure, for certain, sure enough, sure as shooting [informal]

1. Not open to doubt
   "she wanted to go; that was for sure"

See also: beyond a doubt, beyond a shadow of a doubt, beyond doubt, indisputably, indubitably, unquestionable, without a doubt

Encyclopedia: For sure