The phrase "just for heck it" is a colloquial expression that indicates doing something for the sake of doing it or without a specific purpose. Surprisingly, there are no clear antonyms for this phrase as it is a unique idiom. However, synonyms such as "with a purpose," "for a reason," or "intentionally" could be used instead. Alternatively, antonyms for the word "just" could include unjust, unfair, or biased. Similarly, antonyms for the word "heck" could include calm, serenity, or peace. Therefore, using antonyms for individual words in the phrase "just for heck it" could help convey the opposite meaning or intention.