According to the Random House Historical Dictionary of American Slang, the phrase "knocked up," meaning pregnant, first appeared in print in 1830! An 1860 slang dictionary defined the term this way: "Knocked up. ... In the United States, amongst females, the phrase is equivalent to being enceinte." The Oxford English Dictionary traces the expression back as far as 1813 and says it’s of American origin. An OED citation from 1836 refers to slave women who are “knocked down by the auctioneer, and knocked up by the purchaser.”

Come to think of it, Hari, the phrase quite aptly describes what a woman goes through physically in the process of BECOMING pregnant. And it could be a very hard knocking she receives. Not that she doesn't enjoy it ... :-)

It describes the physical relocation of a mans testicles by a front snap kick when his partner finds out he got her pregnant.

"knock (v.) O.E. cnocian (W.Saxon cnucian), likely of imitative origin. Meaning "deprecate, put down" is from 1892. Knockoff "cheap imitation" is from 1966. Knock out "to stun by a blow for a 10-count" in boxing is short for to knock out of time; slang knockout "attractive person" is from 1892. To knock oneself out "make a great effort" is from 1936. Knock-kneed first attested 1774. Command knock it off "stop it" is first recorded 1902. Knocker "door banger" is from 1598; knockers "a woman's breasts" is from 1941. Knock up is 1663 in sense of "arouse by knocking at the door;" however it is little used in this sense in Amer.Eng., where the phrase means "get a woman pregnant" (1813), possibly ult. from knock "to copulate with" (1598; cf. 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]" Source and further information: http://www.etymonline.com/index.php?search=knock+up&searchmode=none "knocked up (idiomatic, slang) "pregnant", typically outside of marriage." Source and further information: http://en.wiktionary.org/wiki/knocked_up