Fifty-fifties is an informal term used to indicate an even split between two options or outcomes. Antonyms for this phrase would be terms indicating uneven or disproportionate splits. Examples of such antonyms include lopsided, imbalanced, and unequal. These words would suggest a situation in which one option or outcome outweighs the other, creating a significant advantage or disadvantage. Alternatively, terms like biased, prejudiced or unfair could also represent antonyms for fifty-fifties, indicating a substantial advantage for one option that is not based on merit, but rather on external factors such as personal preference or bias.