The phrase "keep aft" is not commonly used in English, and so there are no established antonyms for it. However, if we look at the individual words, "keep" and "aft," we can find a variety of antonyms.
For "keep," potential antonyms could include give up, abandon, release, or let go. These words all imply some sort of relinquishing or surrendering.
For "aft," antonyms might include forward, ahead, or in front. These words indicate movement or progress toward a particular destination.
In any case, it is important to consider the context in which these words are being used when determining their antonyms.