What a suspension bridge of a word this is. You want to drive right across it without stopping. It’s not the sort of word one highlights. The repetition can almost mess with stereoptics, causing eyes to cross like when looking at repetitive wallpaper. My eyes want to skate over it and then just bounce right on to the next word without dwelling too long.
But that’s not what we’re here for, is it? No, when you have a word with this kind of structure, even if it’s a word that you would not normally focus on, we want to take it apart. And there’s a specific order in which it must be taken apart.
In English we dismantle words from the edges in (we don’t have infixes, although we absofrickinglutely do have the exception of tmesis). But which edge to start with? If we make it unhighlight+ed we have the problem that something that is unhighlighted is not something you have happened to unhighlight; it’s something you have not highlighted. So un+highlighted.
And then of course we take off the ed. But the next question is, why not highlit rather than highlighted? Indeed we do sometimes see highlit. But that’s the result of a reanalysis. You see, the word highlight was first a noun – a compound made of the adjective high and the noun light: those parts of a painting that have high, meaning strong, amounts of light, meaning light-coloured paint. From that we got the verb highlight meaning to apply highlight literally or figuratively. And when we form a verb from a compound based on a noun, we typically treat it as a regular verb, even if the noun has a related verb that is irregular. We can also add ed to nouns to make an adjective relating to application of the object of the noun (“a sports-jacketed, mustachioed professor”), and in that case it’s always straight addition of the suffix. Unhighlighted is probably formed from the verb, but one can make a case for a noun base.
Well, there might as well be something regular about this word. The spelling is not exactly what some would call phonetic. Oh, we know very well that igh is said as /aɪ/, but the point is that we don’t say it like “igh” – whatever we would say that as. High front vowel plus voiced velar fricative? Or plus voiced stop plus the /h/ sound?
The spelling is that way because, of course, the pronunciation was once like that too. The sound was not always as high and light as it is now. The Old English words were heah and leoht. The pronunciation of the eah and eoh would have been like a southern US gentleman saying air and then clearing a little popcorn husk off the back of his palate.
So the sound changed, and the spelling didn’t keep up. And the meaning expanded, too, as new uses required the word; now, along with talking of highlights in paintings, and of a trip to the gallery being a highlight of a visit to another city, we also talk of using a highlighter to highlight passages in a book – and leave the rest unhighlighted.