Q From Edwin Kiser: We never seem to encounter the word, fro, except when used along with to, as in the phrase: to and fro. What happened to the m that is usually on the end of from to make it fro?
A It’s not that fro is a truncated version of from, but that we have here two distinct and differently spelled words. At one time, we had both of them in the language simultaneously with essentially the same senses, except that fro could also mean “back” in the sense of returning.
They came into the language from slightly different sources at different times: from has been around since the very earliest days of English, having come over with the Saxons from continental Europe; fro seems to have travelled south from Scotland in the early fourteenth century, having probably been borrowed from a Scandinavian language. But they can be traced back far enough that it’s clear they have a common ancestor in a Greek word of the Classical period.
These days to and fro, back and forth, is a fossil phrase that has to be explained in dictionaries because we have completely lost fro in mainstream English. However, fro is still around in Scottish and Northern English dialect, though usually spelled and said fra; it isn’t a dialectal pronunciation of from.