I think the outdated/obsolete nature of terms in many idioms is what makes them charming.

It also makes it pretty clear you aren't using them in a literal sense. If your idioms are too new/unfamiliar and your terminology is too modern/relevant then people might just think you're being literal and not idiomatic.

"Avoiding things like the plague" because people did not actually seem to want to avoid the plague last time around.

Eating a dead horse - not unless you buy the budget beef mince at Tesco

Read like rabbits - rabbits can't read or even hold books

Nip it in the butt - that's sexual assault, you're not allowed to do that any more

Get down off your high horse - you're not allowed to get animals high either

Kids should be shorn and not herd - goats have hair, not wool, and can't be shorn. Their natural tendency to herd is a difficult behaviour to break.

Under the weather - unless you're a pilot you're going to be under the weather most of the time

A blessing in the skies - unless your bishop is a pilot too, that's not going to happen.

Fight off more than you can chew - I don't know where to start with this one,

In my home country they say "Cheap as Onions" (eq in English : Cheap as Dirt)

Looking at the prices on Onions nowadays ...

Oh, man, when it comes to nautical idioms, I could go on all day. There are so many, and they've lost their literal connections so thoroughly, you probably don't even think of some of them as idioms. A few that occur to me immediately:

• taken aback (wind on the wrong side of the sails)
• give a wide berth (extra space for a vessel to maneuver)
• leeway (unwanted downwind slippage of the vessel when in motion)
• above board (on deck, the board being the section of the hull running fore and aft just above the water line)
• onboard (to load onto the deck of the ship)
• go by the board (to fall off of the ship into the water, passing the board on the way)
• hand over fist (two-handed motion to pull in sails, ropes, etc. as quickly as possible)
• high and dry (above the tide line)
• loose cannon (a gun insufficiently secured would cause havoc on a warship when fired, or as the ship rolled in waves)

Honestly, I'm not sure that I want modern equivalents, since many are so rich in imagery.

A stitch in time saves nine.

This refers to a time when clothes were hard to come by and often homemade. If they tore, a small tear could be fixed with one stitch of needle and thread, but if let go too long, it would take many more stitches to repair a bigger rip.

Change your oil (or tires) in time, or change your car.

"Have your cake and eat it too" doesn't really make sense. Cake is not rare or expensive enough that simply possessing it feels good. If you have cake, all you want is to eat it.

The italian version, "you want a full barrel and drunk wife" illustrates the same situation much more clearly.

“Burning the midnight oil”

Don't put the cart before the horse.
Don't submit your post before you write it.

I don't think it's the char/open fire but rather that both pot a d kettle would be made of the same black cast iron

All the horse ones because we don't use horses anymore. There a ton of them.

Don't look a gift horse in the mouth. Who tf gets a gift horse?

Saddle up. Who tf has a saddle?

Hold your horses. Who tf has horses to hold?

Get off your high horse. Who tf has a high horse?

Beat a dead horse. WTF.

Dollars to donuts originated in an age when a dollar could buy a considerable quantity of donuts. Inflation has since reduced the literal interpretation of the expression to nearly even odds.

FYI this can still happen on liquid propane stoves, so not totally lost to time.

Maybe "Kill two birds with one stone", because who hunts with stones anymore?

"Pulling my leg." Meaning you are making something up/joking at someones expense.

It does not have any real connection to the present. The idiom came about in the mid-19th century. There's two competing theories about how it came about. The prevailing theory is that pick pockets, particularly child pick pockets, would pull on people legs to distract their mark. The darker theory is that it was related to pulling someone's legs when they were hanged in order to finish the ordeal if they didn't die from the initial drop.

In Portuguese the equivalent expression is timeless: "o sujo falando do mal lavado" (the dirty one talking about the badly washed).

(to do something) "on your dime," i.e. at your expense - A reference to public coin operated phones, which for a long time cost 10c/a US "dime" coin per call.

Inflation by the late 70s had made the single dime call a thing of the past, but USD20c was still possible, and inflating phone fees in the 80s required increasing amounts of coinage. Public pay phones were decimated when mobile phones gained popularity and are virtually extinct today in the US.

"on your card" might be a more modern version, but cards are slowly disappearing with NFC phone payments.