See mind your own business.

(idiomatic) Alternative spelling of mind one's p's and q's

(idiomatic) To form one's personal opinions and choose one's actions without being governed by the views or choices of others; to be independently minded; to think for oneself.

(informal) Used to express acceptance of something offered to the speaker.

(idiomatic) Extremely bored.

(idiomatic) To clearly understand one's own feelings, intentions, preferences, etc; to know precisely what one wants; to be decisive or determined.

One should put one's mental faculties to good use.

(simile) A remarkably efficient mind; great skill in learning and/or reasoning.

Contentment is more a matter of attitude than circumstances.

(idiomatic, dated) A phrase used at parting.