noun
A person who drives game towards hunters during a hunt. The reading せご is a variant, but せこ is more common.
勢子が山の斜面を叩いて獲物を追い立てた。
The beaters drove the game by beating the mountain slope.
猟師は勢子の合図を待って構えた。
The hunter waited for the beater's signal and took aim.
Rarely used kanji form.
猟師 is the hunter who shoots the game, while 勢子 is the beater who drives the animals toward the hunter.
The exact derivation is uncertain. The kanji 勢 (force, energy) and 子 (child, person) suggest a person who uses force or momentum, fitting the role of a beater.