Mfcttrf

There's nothing wrong with "... get your knickers in a twist". It's the idiomatic expression, and it can be applied to anybody regardless of gender or underwear choice. They are, after all, only metaphorical knickers, not literal ones.

Replacing the nouns in that expression will just make it seem as if you don't know the idiomatic form. Whatever gender-neutral boxes it might tick, it would fail to be good English.

Is this affected by region? I believe I've heard "undies in a bunch" most recently, which has the advantage of carrying the rhythm of the original.

Try this:

I'm not going to get all bent out of shape.

No gender implied at all, and the meaning is the same (to get upset to distraction about something).

1. (of a person) Upset or angry.
   Don't get all bent out of shape—I'm sure she didn't mean to insult you.
   
   You should apologize to Phil before he gets bent out of shape.
   _{TFD Online}

Out of sorts, don't get upset

You could say 'don't get upset', or 'there's no need to get out of sorts about it'.

Note that 'knickers in a twist' is a fairly rude phrase. Saying it directly to someone is always going to be really 'fanning the flames' - not calming the situation. You can't convert a rude phrase like this into something polite without changing its very nature.

Non-rude alternatives are going to need to include genuine care for the person and be calming or assisting the situation.

You may however, harmlessly use the phrase to talk about someone out of their earshot, like 'my boss really got his knickers in a twist about my lateness'. But if he overheard you saying it - that would be baaaad! Sooo. Bad.

Note that in UK English we use the expression for men or women, still with 'knickers'.

Why? Well it's even more funny!

Postscript:

'knickers in a twist' is a way of saying 'I perceive that something, under the surface, unseen, may be troubling you'. You need therapist-level sensitivity, to be able to pull off saying that to somebody without enraging them.

There are some proposals written by a Jacob Weeks on Quora:

Growing up I read a lot of Andy Griffiths books, and my answer is
heavily influenced by that fact. My chosen phrases are:

• Don't get your knickers in a knot


• Don't get your trousers in a twist

Here are some that I have concocted:

• Don't get your slacks in a swivel




• Don't get your jeans in a jerk

I also came up with this one, but I don't think it has quite the same
ring to it:

• Don't get your underwear into an undulation

The first two are too gender-suggestive; the jeans won't work because jerk isn't a gracious word; slacks might work but I think the undulation is funnier, so that's my current best candidate.

I have always liked "I won't get my undies in a bundle". Though, technically, undies usually refer to woman/girl underwear, it doesn't necessarily always refer to female underwear.