Later: runt of the litter
Yoichi, I finally thought of a common phrase somewhat like what you say. Well for an Onikko, not the other one. If you are the shortest sibling, schoolyard language might be "he's the runt of the litter."
This is not exactly like Onikko -- but it is one subset of Onikko!
In certain circumstances, if you were translating a minor character in a novel, you could get away with "runt of the litter" as a quick substitution.
("Runt" is simply the word for an undersized farm animal (say, a pig) or the smallest (say, pig) in a litter. When you use it for a human, it's mildly derogatory, mildly comic.)
In my opinion, there's really nothing for this in English.
(I was fascinated to learn the Japanese terms - thanks for that!)
{By the way. This is a great concrete example of how Japanese (and many other languages) are incredibly less "politically correct" that today's English.}
One thing comes to mind:
"red-headed stepchild" is an unusual often-used derogatory term for - not so much exactly what you ask - but a child that is treated badly, or is really "excluded from" the family, let's say. (A little like, say, Cinderella in the story.) It is highly derogatory (or else, just used wildly humorously), and it is wildly offensive to both red-headed people and adoptees :) So don't use it :)
black sheep is not, really, close to what you're asking, it's more one who is well, a bad person in an otherwise straight family (or used in lighter ways, example all the other siblings get high marks in school)
Regarding the references to postman and similar. If you're wondering what the hell we're talking about...
It is a low-quality standing joke in English that if a child's father is not the mother's husband -- ie, she cheated on him -- then the father was "the postman" or "milkman". indeed, anytime you see someone who does NOT resemble their father, many people would make a low-quality joke regarding "the milkman". For example, they might snicker and whisper "Heh, the postman has been busy around here..."
(That type of humour very much makes me think of [Benny Hill(http://www.wikipedia.org/wiki/Benny_Hill) and the "low quality" British TV humour of the 1970ish, such as the "Carry On..." series.)
{It would be staggeringly hard to explain the "Carry On..." series to someone from Japan, if you're not familiar with it. :) }
{Example: we had a TV series in that time period called "Are you being served?" which is set in a department store. (The title is a (incredibly lame) sexual innuendo.) Now, one of the characters had a cat at home. In English you can call a cat a "pussy", and that is also a slang term for vagina. Now, almost the entire content of the show was based on "pussy" being a pun with vagina. So, the lady in question would say "my pussy was so hungry today," "my pussy really needs a wash" and so on and on and on. ... ! honestly, that was sort of 70% of the entire comic content of the show :) }
Anyway, my point is that the "postman..." reference from the snickering Lads above, was rather in that "Benny Hill" "Carry on.." sort of poor-quality British humour of the 1970s.
{Note that, confusingly, shortly after this in the UK we/they had on TV incredibly subtle, outstanding, upper-class intellectual comedy - heh! For example to the manor born {in this case the title is an exceedingly fine play on words} which contained an unending stream of absolutely amazingly subtle humour in English, eg., Audrey points to a fireplace and says "antiques, He wouldn't know it from Adam."}
Bottom line - there's no term for that in English.
Note - Fumble brilliantly thought of throwback.
That's not a term for Onikko or tobi-ga-taka-wo-unda-ko.
But it's very often used, exclaimed, when you see or are talking about a tobi-ga-taka-wo-unda-ko.
{For example. if we did in fact use the word tobi-ga-taka-wo-unda-ko in English, you'd hear "Wow, she is such a tobi-ga-taka-wo-unda-ko - must be a throwback a few generations!"}
(Note however that Fumble asserts it's for Onikko, me, I've only ever heard it used for tobi-ga-taka-wo-unda-kos.)
neighbors mistake
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