Genitives in -(th)rach
Most Gaelic genitive forms are reasonably close to their nominative forms. One major exception is an apparently weird group of nouns which turn an -air into a -thrach .
Actually, they are weird. So for a change we won't go into the details of why (tied to case system changes between Old Irish and Middle Irish) but simply look at what's going on and how we can spot one of them weirdos.
In modern Gaelic, for starters, they only affect feminine nouns. Add the second clue that they only affect feminine nouns which end in -air and the list of candidates shrinks considerably. That's because by far the most common nouns which end in -air are masculine, where the -air is the agentive suffix i.e. someone who does something: clach » clachair, sgrìobh » sgrìobhadair.
So, let's take a word like iuchair. It's feminine and ends in -air. What happens is that the -ach ending gets stuck on: *iuchair-ach. But because that comes out as a three syllable word (which is a bit long from the Gaelic point of view which likes two syllable words), the second syllable (which was unstressed to begin with) collapses to make way for the -ach. And because the ch is broad [x], the slender r [rʲ] gets assimilated into a broad [r] and we end up with iuchrach.
Now the catch (and slightly annoying thing) is that this is a bit weird even for Gaelic. So, over time, the number of words which do that has decreased. Basically, people played around a bit and settled on a genitive which wasn't as far off. This can be seen in the case of màthair which used to have its genitive as *màthrach (it still does in Irish). But in modern Gaelic this has changed to màthar. Or, a solution was tried with the word cuid which used to have the genitive codach but which nobody really uses any more. Instead they just use the nominative e.g. meud na cuid seo. Oh, plus that, people have also started to switch the gender to masculine to bring them in line with all the other -air words. Machair used to be feminine only but today quite a lot of people treat it as if it were masculine.
So, it all gets a bit messy. But here's a list of some common words that still do, or used to:
|Ones that do|
|And one's that don't anymore|
Yu orait long? as they say in Tok Pisin ...
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