Q From Glenn Dunlap, Texas: Could you please let me know if you have ever made any sense out of the word forthwith? I know it means immediately, but the words forth and with don’t combine to mean immediately.
A They don’t now, but they did once. Forth is one of those words that has gone out of the language except in a number of set phrases. Way back beyond the Norman Conquest it meant forwards. That sense lingers in the phrase back and forth, backwards and forwards.
Another semi-archaic phrase is from this day forth, in which it has the figurative sense of onwards; and so forth includes it in a similar sense; henceforth includes the same idea of time as does forthwith; to hold forth is to talk at length about some subject, often tediously; sally forth is still known.
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.
Much of the idea of forth has been taken over in today’s language by forwards, but that word itself is a contracted compound of forth with the suffix -ward, to go in a specified direction.