Reader CL writes:
I've been using "lengthly" all this time; my mother used it; I believe her mother, an English teacher at a high school in Brooklyn, used it. Today, at almost 33, I saw a wiggly line from a word processor that was my first clue it's not actually a word. Wiktionary says "misspelled form of lengthy."
How did this happen? Is it widespread though (prescriptively) wrong?
Let's start with what seems to have happened. English has a common word-formation process for making nouns into adjectives by adding -ly: kingly, queenly, scholarly, manly, cowardly, weatherly, hourly, yearly, … And English has another common word-formation process for making nouns into adjectives by adding -y: homey, stony, thirsty, icy, rainy, snowy, thorny, fiery, hairy, earthy, …
Both of these processes are at least somewhat productive. If we have an eccentric friend named George, who has just done something characteristically odd, I might say "That was a very George-y thing to do"; and I might also say, with a slightly different meaning, "That was a very George-ly thing to do".
There are a few nouns with well-established derivatives of both types: earthy and earthly, homey and homely, for example. And there are plenty of nouns lacking an established derivative of either sort — laptop, for example. This doesn't prevent occasional lexical creativity:
Our laptops been all loopy & half dead to the laptoply world lately, but WOOOOOOHOOOOO she lives once more "Hallaluyah" & aww im so so happy to be back in the lovely BC land
Xbox 360 Laptop more laptop-y than ever
But as with my George-y and Georgely examples, these are not standard words. Similarly, sunny, scholarly, and lengthy are commonplace, while sunly, scholary, and lengthly are whimsical, poetic, or perhaps just mistaken. Given the default meanings of the -ly and -y suffixes, this is not entirely arbitrary; but there's a bit of a random factor here.
How common is lengthly? There are quite a few examples out there, including some in news sources and books. And some of these seem unlikely to be simple typos, like those in the 12th annual report of the Council on Environmental Quality, which tells us that "The application and approval process is lengthly", and answers the question "When is a lengthly EA appropriate?" with the answer that "Agencies should avoid preparing lengthly EAs except in unusual cases … In most cases, however, a lengthly EA indicates that an EIS is necessary."
There might be alternative universes in which lengthly is ordinary English, and lengthy is whimsical, poetic, or mistaken. But this is not one of them.