@maxtitus sanitation levels have varied, and methods of cleaning one's body have varied, but people have always had means to clean themselves.
Here's the wiktionary etymology:
From Middle English sope, sape, from Old English sāpe (“soap, salve”), from Proto-Germanic *saipǭ,
In other words, in High Medieval England they had a word for "Soap", in Dark Age England they had a word for Soap. The Proto-Germanic word just meant pouring or dribbling, so the concept of soap gets defined in our language during the dark ages, when the Angles and Saxons invaded a civilized place (England) and adopted the practices for themselves. Including the practice of bathing.
It is largely a myth that things were "forgotten" during that time. It is true that a lot of barbarians began adopting civilization, making it appear to later people as if civilization declined, when in reality, its territory just expanded. The economy and population declined badly, and this is why there was a lot less urbanization and thus a lot less architecture, but some of it (see romanesque and gothic cathedrals) was pretty darned impressive.