get your knicker's in a twist (the right side of pond experession)

let your boxers bunch up

get your panties in a wad..

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there are several variations on a theme depending on the local words for underwear.

no doubt even one for jockey shorts (that i don't know/remember right now.
all mean --don't get upset by something so small/trival/easily resolved.

no doubt the expression grows from someone restlessly squirming in a chair about some small problem...

my other obsession