The term "tacking on" refers to adding something to an existing item or list. The antonym for this term can be "removing," which means taking away something from the original item or list. Another antonym for "tacking on" can be "reducing," which implies making something smaller or lesser than the original. Lastly, the term "subtracting" can also be an antonym for "tacking on," which suggests taking away a portion of something. In contrast to "tacking on," these antonyms highlight the opposite action of reducing or eliminating something from the original item or list.