A single-line whip.

(simile) Faithful to a promise one has made.

Alternative form of pick one's battles

To only involve oneself in disputes that one is likely to win. / To only take a stand for a cause that is truly important to one.

Alternative form of show one's true stripes

(colloquial) tired to the point of exhaustion, but still awake and out of bed

(Scotland, idiomatic, dated) To bother oneself; to worry about something.

(idiomatic) To eat heartily. / (idiomatic) To perform at a high level in one's official role.

(idiomatic) Standing erect but not consciously aware of one's surroundings, or only minimally aware, and having little or no ability to control one's bodily actions, as a result of physical injury or exhaustion.

(transitive, idiomatic) To desire with intensity and commitment, to earnestly long for.