It's a phrase written in what is known as "mirror writing" - backwards, that is.
Properly arranged, it is: Þæs ofereode, þisses swá mæg. This is an Old English maxim, found throughout the poëm Déor, and is there used to reference past hardships endured by various heroes and heroines in Germanic history. It could be translated as, "That has passed; this may too."