History of the Offside Rule
One of the problems with internet writing is that useful articles and pages tend to disappear.
Thanks to the Internet Archive, however, we do have access to at least some of the long forgotten works of the past.
For example, take this page, which offered a comprehensive history of the offside law going back to when the rules were first written down at the public schools in the 1810s:
Basically, the original concept appears to have been that all players on the team needed to line up behind the ball. “Passing” was only possible going backwards in those days. Not only is this similar to rugby — it is rugby.
The Eaton College rules of 1847 called offside “sneaking,” and indicate that forward passing was legal:
We do have a complete copy of those rules; see here for a transcription. There is no overt reference to forward passing in the rules, though it’s clear from the context that it was possible.
And then comes the Cambridge rules, as stated above. Those rules also settled on the “three men back” concept.
It’s interesting to note that offside in ice hockey also originally followed the old rugby rules. It wasn’t until the 1920s that ice hockey teams could legally pass forward.