noun
dominant arm; one's stronger arm
Refers to the arm a person naturally uses for writing, throwing, etc. Often used in contexts like sports, daily tasks, or injury.
彼は利き腕を骨折して、しばらく左手で字を書いていた。
He broke his dominant arm and wrote with his left hand for a while.
野球では、利き腕と反対の手でグローブをはめる。
In baseball, you wear the glove on the hand opposite your throwing arm.
利き手 specifically means 'dominant hand', while 利き腕 refers to the whole arm from shoulder to hand.
Compound of 利き (kiki, 'effective, working') + 腕 (ude, 'arm'). The exact historical derivation is uncertain, but the word transparently means 'the arm that works well'.