If it's from the novel Tom Jones, it's a play on words: thwack , onomatopoeia, the sound made when something is hit hard + 'um a variation of 'em, short for them.
It's been a few years since I've seen the move; i.e., heard how Thwackum is pronounced, so I'm not 100% sure on the pronunciation. Let's wait to hear what others have to say.
SparkNotes: Tom Jones: Character List
is the vicious tutor of Blifil and Tom who constantly beats Tom
and praises Blifil. Thwackum, who claims to value Religion above all else, seeks only his own good.
And there's the legendary launderers Rippem and Tearem.