The phrase "tack on" means to add or append something to an existing object or idea. Its opposite term or antonym would be "remove" or "subtract." Instead of adding something, subtracting or removing something would be its opposite. For example, instead of "tacking on" more tasks to an already overloaded schedule, one would "remove" or "subtract" items from the schedule to make it more manageable. In a business context, instead of "tacking on" additional fees, one would "remove" or "subtract" them to remain competitive and attractive to customers. Antonyms for "tack on" ultimately involve taking away, reducing, or erasing, while the phrase itself means to add or increase.