Ah, roger that, thanks!

I see that this is a British English word, so it brings to mind the old saying: "Two peoples divided by a common language."

If it helps, I grew up in England and never heard this word either -- sounds like use outside aviation/Navy is rare.

It's archaic, apparently an abbreviation of opposite number. We might use work buddy today, where colleague is too wooden.

Was pretty common to hear amongst the wartime generation, but has declined pretty rapidly since the 80s. Never once heard it used for "sweetheart" as Webster's seems to think.

Coincidentally, not sure if the brand https://en.wikipedia.org/wiki/Oppo was inspired by that, but it certainly fits.