Reremouse comes from Old English hrēremūs, hrēaþemūs ('bat'), from a base related to hrēran ('to move, stir') plus mūs ('mouse'); literally a 'stirring/fluttering mouse'. The historical chain includes Old English hrēremūs and Old English mūs. Older glosses in the chain include "bat", "to move, stir", "mouse", and "stirring/fluttering mouse", which help explain the semantic range of the modern word. Morphologically, reremouse is best read as borrowing, yielding an English noun shaped by both form history and later usage. In current English it functions as a noun, with the sense "an archaic or dialectal word for a bat (the flying mammal)". related forms include mouse. The result is an English noun whose etymology preserves the older forms while supporting the word's specialized modern meaning.