An idiom is a phrase characteristic of a particular language, that cannot necessarily be fully understood from the separate meanings of the individual words.
A classic example of the idiom is the description of heavy rain in various languages. For example, in Afrikaans it rains old women with knobkerries (clubs).
In Danish it rains shoemakers' apprentices, the Dutch get pipe stems, the Greeks suffer from falling chair legs and the Icelanders receive fire and brimstone. In Catalan it rains boats and barrels while the Chinese have to suffer dog-poo.
In English we often say it's raining cats and dogs. The Germans also find it's raining young dogs. At one time (before stair rods virtually disappeared) the English also suffered from it raining stair rods while rods also fell on the Swedes (along with ladles).
In France and Poland it rains frogs while I seem to recall from my French lessons that nails and taxis were also likely to plummet from the skies!
Instead of taxis the Slovaks find its raining tractors. The Czecks are less ambitious and only receive wheelbarrows.
The Germans say it's raining cobbler boys whilst in Ireland it's throwing cobblers knives. The Welsh get forks along with their knives. The Portuguese get rained on with penknives and, most remarkably, toads' beards. In Spain it's even raining husbands!
Perhaps my favourite is the Norwegian idiom - it's raining female trolls.
And then, with Welsh, we are back to were we started with Afrikaans since it rains old ladies and sticks...