Charles's Wain is an Elizabethan (and earlier) name for the Big Dipper, which seemed to precede the star Arcturus in its course through the night sky. Though originally called Arthur's Wain (literally Arthur's wagon) for this reason, the parallels between King Arthur and Charlemagne in medieval legend conflated the two of them and "ownership" of the plough passed to the latter.
Wikipedia and our own Webbie below both offer a different etymology, citing origins in the Proto-Germanic karlas wagnaz (churl's wagon) being later associated with Charlemagne because of phonetic similarities, but the OED dismisses this as a guess "made in ignorance of the history."
I came across this name while reading Shakespeare's Henry IV, Part 1:
First Carrier: Heigh-ho! An it be not four by the day, I'll be hanged. Charles's Wain is over the new chimney, and yet our horse not packed.