Since, because of the fact that, as in Inasmuch as I have to go anyhow, I'll pick up the book for you, or Insomuch as they are friends, we can seat them together. The first usage dates from the late 1300s, the second from the late 1400s.
insofar as. To the extent or degree that, as in You will become a good pianist only inasmuch as you keep practicing, or He's lost interest insomuch as he has stopped attending church altogether, or Insofar as this is a temporary measure, we can't complain.
[ Late 1500s