a single item of the kind that you are talking about （所提及的）单项物，单个的人 a person who is not married or in a romantic relationship 单身男子（或女子） a person or an animal that is not a twin, etc. （非孪生的）单生儿，单生幼畜
Word Originlate 19th cent.: from single, on the pattern of simpleton.