Webster's Revised Unabridged Dictionary · 1913

Thwack

Thwack (thwak) , transitive verb

[Compare Old English thakken to stroke, Anglo-Saxon þaccian, English whack.]

1.
To strike with something flat or heavy; to bang, or thrash: to thump.
A distant thwacking sound. — W. Irving
2.
To fill to overflow. [Obsolete] — Stanyhurst

Thwack , noun

A heavy blow with something flat or heavy; a thump.
With many a stiff thwack, many a bang, Hard crab tree and old iron rang. — Hudibras