Hmm, I had heard this somewhere before...

But surely it's not how the 'F word', an acronym, originated.

Hmmm, maybe I'm wrong too:

[Middle English, attested in pseudo-Latin fuccant, (they) ****, deciphered from gxddbov.]

Word History: The obscenity **** is a very old word and has been considered shocking from the first, though it is seen in print much more often now than in the past. Its first known occurrence, in code because of its unacceptability, is in a poem composed in a mixture of Latin and English sometime before 1500. The poem, which satirizes the Carmelite friars of Cambridge, England, takes its title, “Flen flyys,” from the first words of its opening line, “Flen, flyys, and freris,” that is, “fleas, flies, and friars.” The line that contains **** reads “Non sunt in coeli, quia gxddbov xxkxzt pg ifmk.” The Latin words “Non sunt in coeli, quia,” mean “they [the friars] are not in heaven, since.” The code “gxddbov xxkxzt pg ifmk” is easily broken by simply substituting the preceding letter in the alphabet, keeping in mind differences in the alphabet and in spelling between then and now: i was then used for both i and j; v was used for both u and v; and vv was used for w. This yields “fvccant [a fake Latin form] vvivys of heli.” The whole thus reads in translation: “They are not in heaven because they **** wives of Ely [a town near Cambridge].”

Yep, wrong, wrong, wrong am I. Ladies and gentlemen, I am pleased to present you with 'The Straight Dope': http://www.straightdope.com/classics/a2_268b.html

bang, batter, beef, bumble, blow off the loose corns, bounce the brillo, dance the buttock jig, do a dive in the dark, flimp, flurgle, foin, foraminate, futz, get one's leather stretched, get one's nuts cracked, get one's oil changed, go bird's nesting, go bush-ranging, go like a rat up a rhododendron, go star-gazing on one's back, have a bun in the oven, have a game in the cock-loft, have a leap up the ladder, have hot pudding for supper, hide the ferret, hide the salami, hide the sausage, hive it, jazz it, knock it off, lay some pipe, light the lamp, lose the lamp and pocket the stake, make her grunt, mix one's peanut butter, palliardize, pestle, pheeze, pizzle, play cars and garages, plow, plug, plook, ram, rasp, ride below the crupper, shoot between wind and water, strop one's beak, varnish one's cane, wet one's wick, wind the clock, and work the hairy oracle

and ain't having a bun in the oven being pregnant?