in one's element; at home / as it should be; perfectly

(idiomatic) to be involved in, to have a hand in

(idiomatic) to be up to no good

(idiomatic) to have everything be working; to have everything be okay

(idiomatic) to be in the picture about something

(idiomatic) to have an ace up one's sleeve

(idiomatic) to have a frog in one's throat

(idiomatic) to buy a pig in a poke

(simile) ruthlessly, recklessly, carelessly, tactlessly

(idiomatic) to not have all one's marbles, (being) a few cards short of a deck, have a screw loose