"Haddest whack" is an informal and colloquial term for hitting or striking someone. However, there are several synonyms that can be used in its place. These include words like "smack," "slap," "punch," and "whip." Each word conveys a slightly different meaning and level of force, allowing for more precise language use in describing physical altercations. Other possible options include "strike," "beat," "swat," and "thump." Ultimately, the choice of synonym will depend on the context and tone of the sentence, as well as the level of aggression or violence being depicted.