A hen is a female bird. In an English idiom relating to a hen's behavior protecting its eggs or young, mother hen can be used as an expression referring to an overprotective attitude.
Leonard McCoy and Spock played a bit of a practical joke on James T. Kirk by omitting his name from a list of personnel assigned to the USS Enterprise's boarding party to the Levaeri V space station, making it appear to Commander General Ael t'Rllaillieu that Kirk was not going to participate in the mission. After commenting to Ael that Kirk rarely accompanied landing parties, they said they could allow him to participate given the circumstances provided an ample armed escort. Amused, Kirk warned the pair that he would have to hold their hands on the mission if they decided to "mother hen" their captain. McCoy in turn warned the captain about the ramifications of the crew seeing their captain holding hands with his executive officer, while Spock wondered aloud how it would be possible to hold hands with a hen. (TOS novel: My Enemy, My Ally)
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