1660s, "arouse by knocking at the door," from knock (v.) + up (adv.). However it is little used in this sense in American English, where the phrase means "get a woman pregnant" (1813), possibly ultimately from knock in a sense "to copulate with" (1590s; compare slang knocking-shop "brothel," 1860).
Knocked up in the United States, amongst females, the phrase is equivalent to being enciente, so that Englishmen often unconsciously commit themselves when amongst our Yankee cousins. [John Camden Hotten, "The Slang Dictionary," London, 1860]
"official routine or formula," especially "excessive bureaucratic rigmarole," 1736, in reference to the red tape formerly used in Great Britain (and the American colonies) for binding up legal and other official documents, which is mentioned from 1690s.
1869; Englishing of vers libre.
But it is possible that excessive devotion to rhyme has thickened the modern ear. The rejection of rhyme is not a leap at facility; on the contrary, it imposes a much severer strain upon the language. When the comforting echo of rhyme is removed, success or failure in the choice of words, in the sentence structure, in the order, is at once more apparent. Rhyme removed, the poet is at once held up to the standards of prose. Rhyme removed, much ethereal music leaps up from the word, music which has hitherto chirped unnoticed in the expanse of prose. [T.S. Eliot, "Reflections on Vers libre," New Statesman, 3 March 1917]
1590s, in reference to the horizontal and vertical lines of soldiers marching in formation, from rank (n.) in the military sense of "number of soldiers drawn up in a line abreast" (1570s) + file (n.1). Thence generalized to "common soldiers" (1796) and "common people, general body" of any group (1860).