vermehren

The original sense of this root was:

big, large

It’s the origin of the words more and most, which for some reason started being used as comparative forms for the now obsolete word mickle.

The root might have a connection to the root *meg-, which was about the idea of great, and which is the origin of a rather large language tree with words like major, mega, maxi and magni- as well as Mister and much.
There’s no evidence for this connection, but it would make a lot of sense.