crook / kroŏk/ • n. 1. the hooked staff of a shepherd. ∎  a bishop's crozier. ∎  a bend in something, esp. at the elbow in a person's arm: her head was cradled in the crook of Luke's left arm. ∎  a piece of extra tubing that can be fitted to a brass instrument to lower the pitch by a set interval. ∎  a metal tube on which the reed of some wind instruments (such as the bassoon) is set.2. inf. a person who is dishonest or a criminal.• v. [tr.] bend (something, esp. a finger as a signal).• adj. inf. (of a person or a part of the body) unwell or injured: a crook knee.DERIVATIVES: crook·er·y / ˈkroŏkərē/ n.

crook. Detachable accessory section of tubing applied to the mouthpiece of brass instr. such as hns. and tpts. to lengthen the instr.'s tube and thus to give it a different basic key. (Players generally carried 10 or 12 crooks.) Natural tpts. or hns., without valves or slides, could play only the notes of the harmonic series, the crook enabling the player to transpose the fundamental note. Thus for a hn.-player, with all parts written in C, to play in D, he would fit a D crook. The introduction of valves from c.1850 almost eliminated the need for crooks. The term is applied also to the bent metal tube connecting the body of the bn. with the reed, and to comparable detachable bent tubes at mouthpieces of cls. and saxs.

crook trick, wile XII; hooked instrument; †claw XIII; shepherd's staff, bishop's staff XIV; bend, curve XV. ME. crōk — ON. krókr hook, bend.
Hence crook vb. bend, curve XII.