Middle English whennes, from earlier whenne (from Old English hwanon, of Germanic origin) + -s3 (later respelled -ce to denote the unvoiced sound)
Strictly speaking, whence means ‘from what place’, as in whence did you come? Thus, the preposition from in from whence did you come? is redundant, and its use is considered incorrect by some. The use with from is very common, though, and has been used by reputable writers since the 14th century. It is now broadly accepted in standard English.