How did 'forthwith' evolve to mean 'immediately'?

Solution 1:

This very query has just been answered by World Wide Words. Here's an extract:

Round the middle of the twelfth century, the phrase forth mid appeared (mid being essentially the same as the modern German word mit, with), later forth with, to go somewhere in the company of other people. Necessarily, if you go forth with others, you go at the same time as they do. It seems this sense of time eventually took over, though the process of transition isn’t very clear, and it’s mixed up with other phrases that also referred to time. Certainly, by about 1450 the phrase had condensed to a single adverb with the modern meaning of immediately, without delay.