The word 'thwart' sailed into English from Old Norse þvert, meaning "across," during the period of Viking influence. Its original, concrete sense described something lying or moving crosswise, a meaning preserved in nautical terminology where 'thwarts' are the seats going across a boat. This physical sense of opposition evolved metaphorically into the verb meaning "to oppose or frustrate a plan or purpose." The journey from a directional term for a physical position to an abstract verb for obstruction illustrates a classic path of semantic shift, where spatial relationships provide the logic for describing conflict or hindrance.

The word is a simplex form. It originates from the Old Norse adverb and preposition þvert, meaning "across," which is the neuter form of þverr, meaning "transverse, adverse."