Here’s a context clues worksheet on en route. Merriam-Webster says it can be used as both an adjective and an adverb, but I’ve only used it as an adverb, and I cannot, off the top of my head, think of how it might be used as an attributive adjective (The en route barbarian checked his iron-age technology, maybe?), though it makes sense (I am en route) as a predicate. It’s probably worth taking a look a this definition from Merriam-Webster’s website, which provides a couple of useful definitions, as well as noting that en route, in terms of popularity, is in the top ten percent of words.
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