The Oxford English Dictionary traces the word clown back to the end of the 16th century with roughly the same meaning as it has today. In the first few quotes it means something like "peasant, uneducated or foolish person", but quite soon after that it refers to "a foolish stage persona, jester".

They also note that it has several contemporary cognates in other Germanic languages, like North Frisian klönne ("clumsy, loutish person"). There are a number of words in modern Germanic languages that sound a lot like this and mean roughly the same thing, like German klumpen ("clod"), Swedish klumpig ("clumsy"), Danish klump klumpet ("clumsy, incompetent person").

Now, as you may have noticed, the English translations I but in the brackets up there also sound a lot like the words they're translating, so it's no big stretch to assume that clown, klumpen, klumpig, klump, as well as the English words clod and clumsy all derive from the same Germanic root.

That's as far back as the dictionaries I looked the word up in goes, but I think that if we use our noggins, we can trace the word back further. In order to do this, we note two things:

1. All these Germanic words have an initial "k" sound. Now, there's a linguistic principle called Grimm's law (after Jakob Grimm, the fairy-tale guy) that basically states that when Indo-European words make the transition into Germanic words, certain consonants change in a regular and predictable way. One of these is that g changes into k, so it's not a big stretch to assume that the Germanic root for these words were once an Indo-European root beginning with a "g".

2. Many of these words have senses (usually old ones) that refer in some way to the concept of "roundness". Take the German klumpen for instance: in addition to referring to a "clod", it can also refer to a "lump" or "clot" of something, like a "lump of clay". And again, look at the English words I just used ("lump" and "clot"), these are also very likely derived from the same source and also generally refers to round things.

If these two assumptions are correct, we should be able to find words from non-Germanic Indo-European languages that start out roughly gl- and refers to round things. Are there any?

There sure are! The most obvious one is Latin globus, the source for English globe, but there's also glutton (which derives from French and ultimately Latin). These words derive from the Proto-Indo-European root *gel-.

In summary: I'll bet anyone here my left nut that the word clown derives from the same source as clod, clumsy and lump, words which themselves derive from a Germanic root referring to a round lumps of something, and which ultimately come from the Proto-Indo-European root *gel-, meaning "roundness, curling up". So suck on that, OED, I just kicked your ass, etymology-wise!

(I just put waaaaaay more effort into writing this comment than I expected to when I started)