Where does “double” come from?

double (Middle French) comes from Old French doble, from Latin dūplus, from Proto-Italic *duplos — double.

Ancestry of “double”, step by step

Step	Language	Word	Meaning
1	Old French	doble	double
2	Latin	dūplus	double (twice as much, or as big)
3	Proto-Italic	*duplos	double