She quit her part-time job to become a professional singer.

professional 由名词 profession（职业）加上后缀 -al 构成形容词，表示"职业的、专业的"。profession 来自拉丁语 professio（公开声明），引申为"需要专门技能的职业"。professional singer 指以唱歌为职业的专业歌手。