The phrase "was sweet on" refers to having a fondness or attraction towards someone or something. Antonyms for this expression would include being cold, disinterested, indifferent, or unmoved. If someone is not sweet on a person, they may be hostile, unapproachable, or distant. In terms of objects or activities, an antonym for "was sweet on" could be disliking, hating, or having no interest in them. To express the opposite of being sweet on something, someone could say they are sour on it or have a bitter taste in their mouth about it.