Well if nobody wants to read the highly relevant post that actually takes things with context...

Sad Cypress is in Part II, Chapter 3.

I looked for an online copy to copy the text more easily, and here is what I found:

Hercule Poirot shook his head with a resumption of his former manner. “A young girl's heart - as I say, so sensitive. An unhappy love affair, perhaps -”

Nurse Hopkins gave a snort. “Girls don't kill themselves for love affairs -not unless they're in the family way - and Mary wasn't that, let me tell you!” She glared at him belligerently.

“And she was not in love?”

“Not she. Quite fancy free. Keen on her job and enjoying life.”

“But she must have had admirers, since she was such an attractive girl.”

Nurse Hopkins said, “She wasn't one of these girls who are all sex appeal. She was a quiet girl!”

“But there were young men, no doubt, in the village who admired her.” “There was Ted Bigland, of course,” said Nurse Hopkins. Poirot extracted various details as to Ted Bigland.

**************************

This is definitely different from my paper copy. Looks like in later editions they didn't use the acronyms.

Thanks for posting that phrase mate, I was playing Roundabout and there was a french phrase which I couldn't recollect later when I wanted to look it up. This is the one! :)

Lucky strike. :-D

OK, I think this is the right solution after all. Few things point at it:
_Unlike S.A. it doesn't contain periods, so not an acronym.
_https://en.wikipedia.org/wiki/It_%281927_film%29#Reception suggests it was in general usage then. So it is possible even a retired army man might use it.

And the meaning does fit well.
I thought so too and that's one thing that made me lean towards military slang. But then, in Sad Cypress it was said by a nurse.
So it looks like it's a case of an army major using a non-military general expression.

Greek text: Translating it Don't think it helps any more, and truly no idea how well versed the translator was in 40s slang. Glad you found an explanation though :)

"You have either got IT or you haven't."

Common phrase that, IT being representative of something that person has, which most do not. Could refer to talent, looks, inteligence etc. Odd that it would have two periods with it though.

"It's it; what is it?"

You want it all, Tiny, but you can't have it.

So it turns out my career in IT has been a complete bloody waste of time...

Oh... it's already solved...

Well I guess I won't be showing you this, then. :\

After reading your comment I became curious:
I wondered what google translator might show if I try to translate 'je ne sais quoi'. I got 'I don't know what'. Which is definately not quite correct in this context...

A woman that has got this' certain something' which attracts men can be described as 'elle a du chien' in French. Which means 'she dog' (so does say google). In my words it means that 'she's got from dog'....

Never trust a translator... and cats...