One of the mysteries of the English language finally explained.
1A plug or stopper which allows the contents of a container to be quickly and automatically released if the temperature becomes too high or the pressure too great; specifically one made of fusible metal, which melts when the contents become too hot, typically used in a boiler.
2A piece of plastic in the shape of an electrical plug which is used to cover a socket and protect against accidental electrocution (especially of young children).
Early 19th century. From safety + plug.
safety plug/ˈseɪftɪ plʌɡ/
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