also: うでき
noun
crossarm; bracket; semaphore arm
A horizontal wooden or metal arm used in structures such as utility poles, railway signals, or semaphore telegraphs. The gloss 'blade' refers to the flat arm of a semaphore signal.
電柱の腕木に鳥がとまっている。
A bird is perched on the crossarm of the utility pole.
鉄道の腕木式信号機は、腕木の角度で進行・停止を示す。
Railway semaphore signals indicate proceed or stop by the angle of the semaphore arm.
Compound of 腕 (ude, 'arm') and 木 (ki, 'wood'), referring to a wooden arm-like projection. The reading うでぎ is standard; うでき is an older or less common variant.