means A plug is a fitting that stops or fills a hole, or the pronged connector that joins a device to an electrical socket — and, by extension, a quick promotional mention slipped into conversation.

from Plug comes from the Middle Dutch and Middle Low German 'plugge,' a bung or peg for stopping a barrel — so the oldest plug was a stopper jammed into a hole. English borrowed it in the 1600s for stoppers of all kinds; the electrical 'plug' is a much later figurative leap, naming the device by the way it pegs into a socket. The 'plug' meaning a promotional mention is American slang from the early 20th century, possibly from the idea of plugging away at something or stopping a gap with advertising — but that link is uncertain.