Onerous means "involving, imposing, or constituting a burden." It typically describes something that is difficult and unpleasant to do or deal with.

// They were assigned the onerous task of post-show cleanup.

// The government imposed onerous taxes on imports.

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"Morton professed joy at relinquishing politics and announced his intention to retire to his country estate, where he would henceforth be occupied with nothing more onerous than straightening out the pathways in his beautiful gardens." — Gareth Russell, The Six Loves of James I, 2025

The story behind onerous is at once straightforward and, dare we say, poetic. But perhaps that's putting the cart before the horse. Onerous rolled into the English language during the 14th century, via Middle French, from the Latin adjective onerosus, "burdensome." That word, in turn, was hitched to the noun onus, meaning "burden" (source too of our word onus, which usually refers to a burden or responsibility). Onus shares an ancient root with the Sanskrit word anas, meaning "cart." So although onerous stresses a sense of laboriousness and often figurative heaviness (especially because something is distasteful, e.g. "the onerous task of cleaning up the mess"), it has a deep connection with a literal weight borne by a person, horse, or other beast of burden.