In Finnish, eggs (as seems to be the case with several other languages). Funnily, egg in singular is often used as slang for penis.
In Chilean Spanish I’ve heard coconuts (cocos)
The saying refers to being manly, as in men being seen as tough, so telling someone to grow balls means basically “grow up and act like a man” (as in these Urban Dictionary entries).
Now we’ve esatblished that “eggs” are nearly universal – I wonder how saying “Brass Balls” (i.e., very brave) works in other languages. In hebrew, they may be made of Iron or Steel, and rarely Copper for some reason, but never brass!
In A Clockwork Orange, one character uses “yarbles” or “yablockos” which is based on the Russian word for apple. Not sure if that was ever actual Russian vernacular, though.
That’s right, cocos is most used, by far. eggs (huevos) may sometimes be used, but rarely.
Exactly. In fact, in Finnish the singular word egg “muna” is used as slang for penis much more often than the plurar word eggs “munat” for testicles. This is very curious usage when compared to other languages, and makes you wonder why this came to be. After all, as many here have said, eggs for testicles is logical, as they are similarly weak and fragile and the form also matches. But egg for penis? There’s no similarity in form, and as a slang word it appears to be very poorly chosen because of the implied weakness and fragility. Perhaps this explains something about Finnish men… 
Huh, the Online Etymology Dictionary has really gone, er, balls to the wall on this one, with an etymology not only of the word “testicle” but of common slang terms for it.
So besides the widely-used “nuts” (including “coconuts”) and “eggs” metaphors, we currently seem to have only two, very culturally specific, foodstuff examples: the British “plums” and the Nahuatl “avocados”.
Historically, it appears that “gooseberries” was also used for testicles.
Apparently there are some contemporary slang uses of “kiwis” or “kiwifruits” in this sense, but I don’t know if that’s really “standard” slang as opposed to just a joke making the rounds. Oh, and “cherries” too; possibly “oranges”? Anybody have any more concrete evidence for these?
Slight correction: Welsh, not British, because the meaning isn’t shared with Cornish or Breton (in the linguistic sense) nor, to my knowledge, with England and Scotland (in the geographical sense).
Of course, I don’t have good resources for Breton slang, so I could be wrong. All I could find was standard Breton (divgell) and Cornish (diwgell), which, other than being feminine nouns in the dual number, aren’t really interesting.
I admit that my only direct source of knowledge of British slang terms for sexual organs is British comedy panel and sketch TV shows. But I’ve definitely heard non-Welsh British comedians on such shows using “plums” to mean “testicles”, so I don’t see how that usage can be described solely as “Welsh slang”.
Should I be saying “British English slang” or “UK slang” or something else instead of “British slang”?
Well, the thread is about multiple languages, so yes, you could have been clearer. The island of Britain has three living languages that have been spoken on the island continuously since medieval times: Welsh, Scottish Gaelic, and English. All are geographically “British,” but only one is linguistically “British” (or “Brythonic”) and that’s Welsh. Then there’s the British dialect of English, British English. So when you say “British” without qualification in a language thread, it’s really unclear what you mean.
In this thread, I was the only one to mention “plums” before you, and I was specifically talking about Welsh. Maybe it’s used in English too, but (a) I didn’t know that and (b) I didn’t say that it was or wasn’t.
(They’re “eggs” in Polish, too.)
“spuds” (as in potatoes) gets used in UK English sometimes too
Curtis Sliwa uses the word “onions,” but I think that’s a Sliwaism.
“soup #5” in English could mean one’s balls in the Philippines.
Hebrew: beitzim, eggs.
Interestingly, a tractate of the Talmud is called Beitzah (its first word), yet in Orthodox study it is usually not referred to as such, because even way back when it could have sounded dirty. I forget what the replacement word is.
Slovenian: Eggs.
In Spanish they may be iron, but it’s more common to use other expressions which of course will change by location: “more nerve than Alcoyano*”, “a face so hard you could use it as paving**”, “bigger eggs than (general) Espatero’s horse***”.
- According to legend, one time this low-level soccer team was losing by an absurdly large amount, yet they went and asked the referee for extra time “to see if we can at least get a set match”.
** Face in this case being closely equivalent to the English cheek.
*** In a statue of the general that’s in Madrid and whose horse does indeed have unusually large testicles.
In Korean we call testicles “fire eggs.” I don’t think we have the equivalent of brass balls. We usually just say “your liver is swollen” instead.
This begs the question: are there common, food-derived slang words for the penis anywhere else in the world? Wiener comes to mind…but any applicant must be in common use, and have a neutral, not a comical / derogatory vibe.
‘Muna’ in Finnish slang is extremely common, but it’s really only in common use when describing someone else’s penis. "He stood there sporting a hard ‘muna’ "… “His ‘muna’ was weirdly-shaped” etc., or when talking about the generic ‘dick’, ie. “I want me some ‘muna’, ASAP”. A man talking about his own junk uses other terms, which are myriad in Finnish. It’s not all well and good to think of your penis as a ‘muna’, after all.