From Middle Dutch plugge 'bung, peg' — the electrical sense dates from 1883.
A piece of material used to fill a hole; also, a device for making an electrical connection.
From Middle Dutch plugge 'bung, peg,' from Proto-Germanic *pluggaz. The word arrived in English surprisingly late — prior to the 17th century, English used 'stopper' or 'bung.' The electrical sense dates from 1883, and the promotional sense ('give it a plug') from 1906. Key roots: *pluggaz (Proto-Germanic: "peg, stopper").