means A liquid you swish around your mouth to freshen breath and kill bacteria, then spit out without swallowing.

from A plain compound of "mouth" and "wash," both rooted in Old English ("muth" and "wascan"). The thing it names is far older than the word: ancient peoples gargled wine, vinegar, and herbal brews, but the snappy English term for a bottled rinse is a much later, no-nonsense coinage — your mouth, washed, exactly as it says on the tin.