"He is sly as a fox."
Translation:Er ist schlau wie ein Fuchs.
Colloquially, schlau can also be used as any form of smart/intelligent, but in this idiom it's intended as savvy, sly, cunning. Or foxy....
Often "schlau" has a negative connotation, like hiding one's real intentions, pursuing only one's own goals, while "klug" is rather meant as sage, wise, with a positive connotation. Also this can change, see "neunmalklug", "altklug".