Gender inspecific form of Sir or Ma´am?
As in "you sir are a genius" or "you ma'am are a genius"
What do we use for NB's?
maestro (can be mildly ironic)
My dear (for affectionate emphasis)
Guvnor to go full British
It's kind of a though one!
@quixoticgeek actually... I think "my friend" is probably the simplest and most universally useful.
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@ashtardeza Does that not imply a level of informality that may not be appropriate?
@quixoticgeek probably differs per culture, but I have seen it used to basically mean "I have good intentions towards you."
To geek out a little: most forms of address come down to "my lord" and "my lady", at least in Germanic languages. In English the male version is a weird exception, but madam is just my lady in French.
So I'd look for a form of my plus something descriptive of the relationship to the person.