Wiktionary
have another think coming
alt. To be deluded, to be mistaken; to need to rethink something one has determined; to need to reconsider one's plans or expectations. vb. To be deluded, to be mistaken; to need to rethink something one has determined; to need to reconsider one's plans or expectations.
Usage examples of "have another think coming".
So if you think you're leaping out of bed this minute, you'd better have another think coming.
They may show you that you have another think coming-that there is some limit, after all, to your vaunted influence.
She says if you think she'll meekly go away, you have another think coming.
They may think we're stupid animals, but before you two and the Triplanetary Patrol and the Service get done with them they'll have another think coming.