On Sat, 1 Aug 2015 18:46:27 +0000 (UTC)
John McCoy wrote:

A "way" is a road or path. So "by the way" means something
which is beside the main path of the conversation.

could say by the wayside and then it is more descriptive

According to the OED, you're precisely right. They date the
expression to 1556.

easy guess since so many sayings came out of that period

BTW, one of my pet peeves is people who confuse "way" as in
"under way" with weigh as in "weigh anchor". Doesn't annoy
me as much as the people who confuse break and brake, tho.

not seeing that much but maybe not looking that hard