As many schoolboys and students know, unintended innuendo can be found in the most unexpected of places and can lend a certain mischievous serendipity to otherwise dull lessons and lectures. One major source of linguistic amusement is transatlantic confusion. This was recently driven home to me by the following passage from an Irwin Shaw story: "Across the street, on the public athletics field, four boys were shagging flies."

My confused yet amused eyes tripped and staggered over the sentence, but made no sense of it, retraced their steps several times, then sat on the kerb of the full-stop, under the shade of the quotation mark, and scratched their chin in bemusement. Boys shagging flies? Not only is that physically impossible, but why on earth would a celebrated American writer, working in a more decorous pre-Monty Python and Little Britain age, stoop to such crudity? It had to be something else.

Some sleuthing around and further research – the OED, Google and a couple of American friends – cracked the mystery. Rather than implying that a group of young lads were attempting intercourse with insects, the sentence was actually about baseball and catching fly balls.

As George Bernard Shaw once, rather hyperbolically, claimed, Brits and Americans are "divided by a common language". And examples abound of confusing word usages, especially when it comes to slang and popular colloquialisms, not to mention regionally within each country.

Given the growing transatlantic familiarity in the age of the internet and saturating mass media, especially the British familiarity with Americanisms, confusion is receding, but it can still occur. English slang that might confuse Americans includes: the exclamation "bugger", "cowboy" (as in unscrupulous trader), "con" (as in con artist, not convict), "fag" (as in cigarette), to "fancy" (ie find someone attractive), to be "pissed" (as in drunk), etc. Given that we've grown up with American pop culture, most mainstream Americanisms are very familiar and even many obscure local expressions have made it across the Atlantic. But hearing references to "fanny bags" and someone showing a lot of "spunk" can't but elicit a knowing smile from a Brit.

Given the length and breadth of that language ostensibly known as English, the geographical differences don't end there. Although Australian English is, in many ways, quite similar to British English, with perhaps more borrowing from American, there are still significant differences. The first time an Australian friend told me that he felt "crook", I wondered what crime he believed he had committed. What he meant was that he had been feeling ill or unwell.

Of course, meanings do not only change across space, but also across time, in a phenomenon known as semantic shift. Among the most popular and best-known recent examples are "gay", ie happy and carefree, and "queer", ie odd or unusual. In fact, such is the way of things, that a "gay man" once referred to a womanising bachelor and a "gay woman" was a prostitute. Moreover, though gay lib may have really taken off only in the 1960s but before that we had the "Gay 1890s", without a gay pride parade in sight.

As for "queer", which has been appropriated as a term of pride by gay people, long before Britain came out of the closet, it had "Queer Street", where people in financial dire straits figuratively lived, and someone "feeling a little queer" was not touching up anyone, but was, instead, under the weather.

Going even further back, things get really weird! Weirdly enough, if you though the word "weird" was relatively new, think again. It was used half a dozen times by Shakespeare, at a time when it meant possessing supernatural powers. Its modern meaning was coined by the English Romantic poet Percy Bysshe Shelley in the early 19th century. In fact, umpteen apparently familiar words in Shakespeare have quite different meanings today. For example, when one of the witches in Hamlet says, "You all know security/ Is mortals' chiefest enemy", security here means complacency and not safety, which would be nonsensical.

Over time, many words go through elevations in their meaning, or they fall from grace, or they end up meaning the complete opposite of what they originally signified. For example, "knight" simply meant "boy or servant", while "gentle" meant of high birth, hence "gentleman". Girl meant any young person – "gay girl" meant girl, not lesbian, and "knave girl" meant boy – while "man" meant any person, regardless of gender.

Once upon a time, people who were "awful" (ie deserving of awe) and "silly" (blessed and happy) were admired, and people who were "brave" (ie cowardly) were looked down upon. And if you were "fond" of someone, you found them stupid and silly, and if you found someone "cute" that meant they were bow-legged. If all this is a bit confusing, don't "worry", especially since, in medieval England, the word meant to strangle or choke someone to death.

Semantic shift is occurring around us even as we speak and, in the future, words may take on radically different meanings to the ones they have now. Today, in jest, we may say someone is "bad", meaning good, "wicked", meaning cool, or "fit", meaning attractive. But future generations may have no other meaning for these words and may conclude that "survival of the fittest" means that only the beautiful live on.