If the idioms and swears are unfamiliar to the players, then they will feel unfamiliar to the characters. Use weird ones when the players are in a new or foreign region to make it feel more like they are elsewhere. Otherwise, use them in sci-fi and fantasy to remind the players of the world and its lore.

Idioms for unfamiliar regions

Heavy Rain	Death	Anger (ironic)	Not very much
cats and dogs	kicked the bucket	calm as a lake	two shakes of a lamb’s tail
tables and chairs	sharpened the wrong knife	happy as honey	quick as birth (ironic)
axes are dropping	brushed it over	sweet as apples	bee’s feet
wheelbarrows	sneezed the lot	skipping twice over	light-by-thunder
stream of mallets	bounced it twice	pounds in	count to one twice
raining frogs	had half again	flute, fiddle and free	a cricket’s fart
ghost rain	slept over and under	clear as nightingales	a life’s taxes (ironic)
old ladies and sticks	moved aside	thirsty, hungry and sleeping	ladder’s down
poured from a bucket	red and yellow	honeymooning	three pints gone
boats and barrels	dropped it all	bunny eyed and bunny eared	top to top and half over

If challenged on the origin of these idioms, the locals don’t know why they say them, they’ve just always been said like that. Just like in real life.

Swears

Unproved theory: most swears relate to poop, willies, sex or gods

Extension of unproved theory: most poop, willy and sex swears are ‘four-letter-words’ and most ‘four-letter-words’ are poop, willy or sex swears.

In Farscape they say frell.

In Gaunt’s Ghosts they say feth.

In Battlestar Galactica (2004) they say frak.

Some of those are five letters long but they fit the format of a four-letter-word

Four-letter-word generator

Roll 3d8 to get a random start, vowel and end of a four-letter-word.

d8	start	vowel	end
1	f	a	ck
2	c	e	th
3	kr	i	ss
4	pr	o	zz
5	sh	u	t
6	tr	ee	p
7	ch	oo	st
8	z	ai	ll

Some results might be real words but you could just reroll them with little effort.

You might want to roll to decide what the word represents, or you could choose based on how it sounds.

d4	Type of swear
1	poop
2	willy
3	sex
4	something else

Swears relating to gods and such

If a swear invokes a name, I’m going to call it a curse from here on.

Curses in English are mostly about invoking the christian god’s name.

Cursing by literally saying/shouting the name can be replaced with the names of gods in your setting. If your setting is devoid of gods you can use a legendary figure.

Sometimes curses call upon an aspect or item of a character.

In Harry Potter some characters say Merlin’s Beard!

Marvel’s Thor similarly sometimes says Odin’s Beard!

An Arthurian character might exclaim By Excalibur!

I can’t really give you a random table that will work for your setting. You just need to spend 2 minutes listing a few possible curses and stick them somewhere in your notes.