I actually have a whole book about Welsh idioms .... although it's also about prepositions as well..... it's called Y Geiriau Bach ( Idioms for Welsh learners) and it's by the rightfully celebrated Cennard Davies.... an ex-headmaster I believe.
Here's a couple of examples of what it does ....
Dan sylw ...... it really means " in question" .... and you get an example ..
Ar y diwrnod dan sylw roedd y ferch yn Llundain.
On the day in question the girl was in London.
Ar y galeru ... it means "dead" here's his example .....
Fe fyddaf i ar y galeri ymhell cyn diwedd y canrif.
I'll be dead long before the end of the century.
That's pretty useful stuff. What a grand book!
But the real fun of such "phrase books" is what they reveal about the author!
[ Slight diversion here ..... I've got loads of phrase books for various languages and some of them are really really peculiar! One of them is full of preposterous sentences you would never ever say, and another makes just about every phrase full of sexual innuendo or worse! ]
Now I've mentioned that Mr. Cennard was a headmaster.... well well ... lots of his example sentences are about headmasters !
... e.g. ... To illustrate the idiom " heb bilo wyau" = to pull no punches..... we get ...
Dywedodd Alun wrth y prifathro beth oedd ei farn am yr ysgol heb bilo wyau.
Alun told the headmaster what he thought of the school without pulling his punches.
Strangely though, they don't get a mention at all until page 52! What are his other obsessions? Well, There's lots of thieves, loads of deaths, stacks of crap weather, quite a few rubbishy people, violence, dogs, rugby, .. lots of rugby actually, quite a few foxes, some fires, the moon crops up in places,dogs, funerals .. and a few weird ones. Let's take a look at a few.......
[A] To illustrate ar led = going around, "abroad" ....
Mae si ar led ei fod e wedi marw
There's a rumour abroad that he's dead.
[B] To illustrate ar drawiad amrant = in a flash, in a wink ....
Diflannod y cadno o'n golwg ar drawiad amrant.
The fox disappeared from our view in a flash!
[C] To illustrate ar ben set = "all up"
Roeddwn I meddwl ei bod hi ar ben set arnom, ond fe daeth y frigâd dân.
I thought it was all up for us, but the fire brigade came.
[D] To illustrate yn llewys ei crys = in shirtsleeves.
Byddai'r athro'n dysgu bob amser yn llewys ei grys.
The teacher always taught in his shirtsleeves.
More of this educational silliness later ... but now ...
Music time ... and this is the excellent DeMask with Ben dy Hun ..... one of many Welsh pop videos on a bandstand !