I/I’ll bet! Definition 1 of 2Pro Tip 1/3Express Certainty Use it when you're almost sure about an outcome, showing a strong belief. I bet the teacher will assign homework over the weekend. Pro Tip 2/3Colloquial Assurance It's informal and used in everyday conversation rather than in formal writing. I'll bet the new restaurant is packed tonight. Pro Tip 3/3Predicting Behavior Often used to predict someone's actions based on what you know about them. I bet she's going to be late again.