1. an absolute certainty or not a problem
trust me, it's a sure thing
This informal expression is quite versatile. It can be used to say that something is a guaranteed success, similar to 'nailed it,' to express enthusiastically that you will certainly do something that has been asked of you, in a similar way to 'of course!' and also to let somebody know that something is/was not a problem, 'sure thing' is usually said in response to a 'thank you.' Its versatility means that it is commonly used in both a social and professional context.