I think this mix up, another one that’s not a classic troublesome pair, in that one is correct and one is incorrect, comes from people writing down what they think they’ve heard. This has come up with Here! Here! vs. Hear! Hear! and would of vs. would have, too. I’ve started noticing this one a lot on Twitter, too, so it might be that people just stick down what they think it might be when typing in a hurry.
Incorrect: on route. Correct: en route
The incorrect usage does make sense in a funny kind of way, and it sounds the same too, but, it really isn’t!
So, as we probably know, it means “on the way” to or from somewhere. It comes from the French – 18th Century French, so it’s been around for long enough that it’s stuck and isn’t likely to be that pliable. You can use on route, but only in a very specific sense, when talking about named roads in places like America. And then there’ll be a capital in the middle, and you’re not using it in precisely the same way. So, “On Route 66 I found a lovely motel” – fine. But in all other cases: “I was en route to Chicago when I happened upon a charming hotel” – also fine.
This also applies to phrases like en masse. If there’s an “on” sound and the other word looks like it might be French, check it!