A cantankerous person is often angry and annoyed, and a cantankerous animal or thing is difficult or irritating to deal with.

// Although the former postman was regarded by some townspeople as a scowling, cantankerous old coot, he was beloved by neighborhood children, to whom he would regularly hand out butterscotch candies from his front stoop with a twinkle in his eye.

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“The film ‘Hard Truths,’ which opens in New York on Friday and nationwide in January, centers on [Marianne] Jean-Baptiste’s Pansy, a cantankerous middle-aged woman who spits venom at unsuspecting shop assistants, bald babies, her 20-something son Moses (Tuwaine Barrett) and her dentist, among others.” — Simran Hans, The New York Times, 9 Dec. 2024

A person described as cantankerous may find it more difficult than most to turn that frown upside down, while a cantankerous mule/jalopy/etc. is difficult to deal with—it may not turn in your desired direction. It’s been speculated that cantankerous is a product of the obsolete word contack, meaning “contention,” under the influence of a pair of “difficult” words still in use: rancorous and cankerous. Rancorous brings the anger and “bitter deep-seated ill will” (as rancor can be understood to mean), and cankerous brings the perhaps understandable foul mood: a cankerous person suffers from painful sores—that is, cankers.