Refers to a male domestic servant, typically in a historical or literary context. Not commonly used in modern everyday Japanese; 使用人 or お手伝いさん are more common for general domestic help.
When he was young, he worked as a houseboy in a large mansion.
時代劇では、家僕が主人に忠実に仕える場面がよく出てくる。
In period dramas, scenes of a manservant faithfully serving his master often appear.