Not to give a hoot / Not to care at all
Literally 'not to care a cumin seed'. Used when someone is completely indifferent to something. Comino can be replaced with pepino, bledo, or pimiento.
What other people think doesn't matter to me at all.
—Doesn't it bother you? —I don't give a hoot.
You can say whatever you like to him, he doesn't care at all.