I got'ch!
In Classical Latin, the sound we make from the "w" (as in water) was represented by a letter that looked like "v."
Through the years, the sound that the letter "v" changed to the modern sound - like vapor.
To distinguish the sound of "w" from the "v" AND the up and coming letter-sound of "u," a double form of "u" was taken to represent the original Classical Latin "v" sound which was written as "uu" - which is two of those "u" symbols called "double-u."
