ingrained habit; second nature
Refers to a habit so deeply learned that it feels like an innate part of one's character. Often used in the proverb 習い性となる (habit becomes second nature).
My morning jog has already become second nature.
Since habit becomes second nature, let's develop good habits.
Compound of 習い (learning, habit) and 性 (nature, character). The exact historical derivation is uncertain, but the word is conventionally associated with the idea that learned behavior becomes part of one's nature.