Thwack is an instant-death spell that targets a whole group of enemies, stopping the heart (or nearest approximate). Characters that take up the paths of Priests and Sages are the only humans that learn the whack line, giving the impression of the spell family being a case of "forbidden" magic reserved only for the pious.
Priests and sages will learn Thwack at level 28, and it costs 7 MP to cast.
Kiryl learns Thwack at level 24 and now costs 7MP to cast, remaining so until IX.
Thwack is learned by advancing to rank 7 of the Priest vocation.
Thwack is learned by reaching the first rank in the sage vocation in the original and 3DS remake.
Angelo learns Thwack at level 22.
Thwack is learned by priests at level 34, and costs 10MP. It works off of the priest's Magical mending, raising the base accuracy from 50% once the stat hits 100 points, capping at 100% effectiveness (barring resistance) at 999.