The noun ammunition is derived from the same source as the noun premonition. However, despite the common root, while ammunition may not be a true opposite of premonition, it is comes about as close as it can come without actually being a precise antonym.
Whereas a premonition is a strong feeling that something, particularly something unpleasant, is about to happen, an ammunition is a dead-certain feeling that something, particularly something pleasant, hasn’t got the slightest hope of ever happening.
Then again, if the gist of the premonition is negative, as is typically the case, and the gist of the ammunition is positive, as is typically the case, then the results of the premonition and the ammunition are not all that different in quality. If you’re premonition or ammunition proves to be accurate then you will be in a worse state than if it didn’t. So, maybe the two words aren’t so far apart after all.