"having length" (especially "immoderately long"), 1759, American English, from length + -y (2). Until c. 1840 always characterized in British English as an Americanism.
This word has been very common among us, both in writing and in the language of conversation; but it has been so much ridiculed by Americans as well as Englishmen, that in writing it is now generally avoided. Mr. Webster has admitted it into his dictionary; but as need hardly be remarked it is not in any of the English ones. It is applied by us, as Mr. Webster justly observes, chiefly to writings or discourses. Thus we say, a lengthy pamphlet, a lengthy sermon, &c. The English would say, a long or (in the more familiar style) a longish sermon. [John Pickering, "A Vocabulary, or Collection of Words and Phrases Which Have Been Supposed to be Peculiar to the United States of America," Boston, 1816]
Related: Lengthily; lengthiness.