For most native speakers, this is usually a pretty easy question to answer, as even though the two concepts are quite similar, they’re still pretty distinct.
For learners of English though, the distinction between the two is a subtle one, a situation not helped by the fact that other languages don’t always make a distinction between the two concepts. It’s therefore quite common in the English-language classroom to hear people say they had a funny evening, or thank you for a funny lesson (though sometimes that one’s true).
The understandable confusion between these two words got me wondering how they came to be separate words in English.
First came fun, in the 16th century, as an adjective meaning foolish or silly.
About a century later, to fun as a verb came into usage. To fun someone. It meant to cheat, I suppose because to cheat someone makes them look foolish or silly.
From this, the meaning softened to to trick, and then to have some amusement or sport, which is the meaning it developed when it came to be used as a noun in the 18th century.
At this stage then, fun existed, as it does now, as a noun and an adjective. When people wanted to use a synonym for humorous then, it made sense to create an adjective from the noun fun. Fun though, as an adjective, in the 18th century, still meant foolish or silly. A suffix was needed then, and what better than -y? As a suffix to form an adjective, -y could be used to mean full of or characterised by, which we can see in words like juicy, thirsty or crispy.
The original sense of fun meaning to cheat still lingered on in funny though, which is why we still use it in a negative way, e.g. There’s something funny about him.
All well and good, but funny was quite specific, meaning humorous as it did, and in the middle of the 19th century people obviously felt there was a need for a simpler, shorter synonym for enjoyable or diverting. Funny was too specific, so why not simply use fun as an adjective? Things came full circle then, with fun, which began life as an adjective in the 16th century, becoming an adjective once more.
Fun came before funny then, which isn’t too surprising, but in terms of their present meanings, funny actually came before fun.
Funny, isn’t it?