A yak is a big animal with horns and hooves that's native to Central Asia. A secondary, informal meaning of yak is "to chatter on and on." If you live in the Himalayas, you may spend a lot of time yakking about yaks.
Yaks are relatives of cows and oxen, and in many parts of Tibet it's common to drink yak milk, eat yak meat, ride on yaks, and even race yaks. The English word comes from the Tibetan g-yag, "male yak." Yak can also mean "noisy chatter" or, as verb, "to chatter noisily and at length." This meaning is a variant of the verb yack or yuck and is unrelated to the word that denotes the animal.