I’ve seen more than usual of this word recently thanks to discussions on points of grammar. In general, those who use it – to label, for instance, a use of you and I where you and me is technically correct – mean by it something like “grievously cringeworthy” (or, more to the point, “yuck yuck yuck I hate it I hate it I hateithateithateit!”).
And certainly that’s the flavour of it, isn’t it? It has a feel of grievous – with that growling /gr/ that bespeaks vigour, aggression, anger, or emphasis, from mad dogs to Tony the Tiger, descending to gross and grim and ascending to great and grand – but the extra /i/ at the beginning gives it even more energy, forcing the sound at high pressure past a tense tongue (I remember as a kid saying jee creeps when I was frustrated). And the affricate /dZ/ in place of the fricative /v/ gives it a bit more chewiness and perhaps an echo of cringe.
So it’s hardly surprising that it is commonly followed by violation(s), error(s), abuse(s), and offense(s), along with example(s), case(s), conduct, and behaviour. The word it’s most commonly seen with is most – if one is going to be emphatic, why not go all the way? The world may be crowded with egregious things, but the one you have your eye on is the most egregious of its sort!
And well that an egregious thing should stand out from a crowd – but a crowd of egregious things seems odd indeed. Why? Because the greg in egregious is a Latin root meaning “crowd” or, more accurately, “flock”. We see it in congregation and gregarious. The e is the same as in e pluribus unum: it means “out of”. So something that’s egregious is a standout – in a bad way.
But it wasn’t always a bad way. The Latin etymon, egregius, meant “excellent, eminent, outstanding” – outstanding in the good way. And that is how it entered English in the middle of the 16th century: to mean “preeminent, distinguished, outstanding”, all those good things. Uses of it in this sense can be found in works as late as the mid-19th century, for instance by Thackeray in The Newcomes: “When he wanted to draw… some one splendid and egregious, it was Clive he took for a model.”
But a mere third of a century after it had shown up in English in the positive sense, it was already being used in the negative sense we know now – the photo negative of the original sense. Shakespeare used it in the negative sense: “Egregious murtherer,” in Cymbeline. But Shakespeare also used it in a more positive sense: “Except… thou do give to me egregious ransom.” Which suggests that its use at the time was more in the line of exceptional: capable of being strongly negative or strongly positive, but either way strong.
Well, not now; now the tone does not need to be specified. The positive sense is obsolete and forgotten. In fact, the tone is eclipsing the remainder of the sense; the idea of exceptionality or salience is being drowned out by the basic point-and-scream response it evinces. (Come to think of it, it rather does sound like he screeches, doesn’t it?) And so a very common error of usage comes to be described as egregious. So what term is left for truly outstandingly bad instances like, say, Thou may giveth it to I?