Iets onder de knie hebben

Literally: To have something under the knee

Example: “Na drie maanden oefenen heeft ze de salsa onder de knie.” — After three months of practice, she has salsa down pat.

English equivalent: To get something down pat

Origin: From wrestling, as Onze Taal explains, quoting Carolus Tuinman's proverb collection of 1726: getting someone under one's knee meant subduing them entirely, "taken from wrestlers who have thrown their opponents to the ground." It was said first of people and countries brought under someone's power, and only later of a skill one has mastered.