After-thought is defined as a thought or idea that occurs later or is added as an addition. The opposite of after-thought would be a forethought, which refers to thinking or planning beforehand. Antonyms for after-thought could also include premeditation, preplanning or preconception, as these words indicate that something was considered prior to acting. Another antonym would be consideration, meaning that attention was given to something ahead of time. Opposites of after-thought may also include reflection, contemplation, or deliberation, which suggest that thought was given to something over time, rather than as an afterthought.