Whence means from where, in old or formal use. Whence he came, return whence you came, whence the confusion.
It belongs to older writing and to deliberate formality, and it is one of a small set with whither, meaning to where, and whither's partner hither. From whence is technically redundant, since whence already contains from, though it appears throughout the King James Bible and Shakespeare and is therefore hard to call wrong. Note whence also introduces a consequence: whence the difficulty.
1adverbwhenceFrom where, in old or formal use.
“The travellers returned to the village whence they had come.”
2adverbfactFrom which fact, introducing a consequence.
“The train was cancelled, whence our late arrival at the meeting.”
Too few real examples of this word to estimate which sense is commonest.
whence is an adverb of origin.
Old English hwanon plus an adverbial -s, related to when. Whither, hither and thither were built the same way and have died at about the same rate.
Proto-Indo-European
*kʷ-
Old English
hwanon
Middle English
whennes
English, today
whence
From Middle English whennes, from Old English hwanon (with adverbial genitive -s), related to hwonne (whence when). Analyzable as when + s.
Etymology from Wiktionary, CC BY-SA