Let's take this apart. Let's start with golchi as that is the base verb. The first meaning in GPC is
Glanhau (dillad, llestri, &c.) â dŵr)
which means 'clean (clothes, dishes, etc.) with water'. So basically it is any cleaning process that uses water. Clearly the actual process would vary according to many factors, including what you were washing, and the technology available (which will have changed a lot since the word was first recorded in the 13th century.
Then we add the ym (which makes the g disappear by soft mutation). This means you are doing the golchi to yourself. The comment about technology is clearly important since this word has also been used since at least the 13th century and showers did not become available until the 19th century, or popular until the 20th century.
So I think we can be confident that it means 'get yourself clean using water and whatever technology you choose from what is available'.