Where does the term "motherboard" come from?

This was asked on the English.SE. The most popular answer offers:

It's called a motherboard because it is the main circuit board in the computer, and it can be extended by plugging other circuit boards into it. These extensions are called daughter boards. Wikipedia suggests that historically a "mainboard" was not extensible in this way, hence the need for different terminology. Many computer terms use human or biological words as metaphors.

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