Imperturbable describes someone or something marked by extreme calm; such a person or thing is very hard to disturb or upset.

// The imperturbable captain did not panic when the boat sailed into the path of a violent storm.

// Nothing disrupted the contestant's imperturbable focus.

See the entry >

"The thick heat is not letting up after a long stretch of nearly-90-degree-days, though the crowd has not seemed to notice. Instead, these thousands of people emanate a truly imperturbable energy as they get to see gospel legend Mavis Staples for free." — David Cohn, The Daily Californian (UC Berkeley), 13 Oct. 2024

Imperturbable is a bit of a mouthful, but don’t let its five syllables perturb you. Instead, let us break it down: this word, as well as its antonym perturbable, comes from the Latin verb perturbare, meaning "to agitate, trouble, or throw into confusion." Perturbare comes in turn from the combination of per-, meaning "thoroughly," and turbare, meaning "to disturb"; unsurprisingly perturbare is also the source of the English verb perturb. Other perturbare descendants include disturb ("to destroy the tranquility or composure of") and turbid ("thick or opaque with or as if with roiled sediment").