sure thing

WordReference Random House Unabridged Dictionary of American English © 2025

sure′ thing′, 

1. something that is or is supposed to be a certain success, as a bet or a business venture:
   He thinks that real estate is a sure thing.
2. something assured;
   certainty:
   It's a sure thing that he'll refuse to cooperate.
3. surely;
   for sure;
   O.K.;
   roger (often used as an interjection).
• 1830–40, American.

Collins Concise English Dictionary © HarperCollins Publishers::

sure thing noun

1. something guaranteed to be successful