See also: crewel needle.
Source: Shorter Oxford English Dictionary: 'Crewel'.
GVE
Crewel thread is a fine, loosely twisted worsted (wool) thread used for crewel embroidery. Modern crewel wool is normally a fine, single or 2-plied thread. According to the Shorter Oxford English Dictionary, the earliest written example of the word crewel dates back to AD 1494, but the exact origins of the word are unknown.