You may well be right (you usually know all the answers before you ask), but I would be surprised if the expression is used much (in the UK at least). Personally, I would use a genitive form and say my wife's uncle.
But maybe that's just me.

Keine Sorge, tomtom, der Begriff ist auf Deutsch genauso besch....ränkt genutzt.

Vielleicht besteht ja Hoffnung, dass wmw allmählich die Begriffe ausgehen (aber dann fragt er halt wahrscheinlich nochmal, was er schon vor 2, 3 Jahren gefragt hat) ...

Schwiegeroheim :)

Es gibt übrigens auch den "Schwiegerschwager", auch "Schwippschwager" genannt.

Im Gegensatz zum beschwipsten Schwager?

Full Definition of UNCLE-IN-LAW

1 : the husband of one's aunt
2 : the uncle of one's spouse

http://www.merriam-webster.com/dictionary/unc...

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#1: No, it's not just you. I wouldn't use it either and have never heard it used (in BE), whatever a dictionary may say (which, as you say, wmw probably knew in advance, as usual).

So AE, BE, or German, we all are in agreement that it should be avoided in favor of wife's uncle or husband's uncle.

The only reason I can see ever to use it might be as an explanation of a word in a third language, with a comment such as 'which literally means ...'

Re #5:
For 1: For most people their aunt's husband is their uncle. I have never known anyone call or refer to the husband of their aunt in any other way. If it was important for someone to know the precise relationship, they might add "It's my mum's/dad's sister's husband."
For 2: see #1.

The "in-law" is only to show that it is not a blood relation, which is normally only important with the immediate family - probably due to the rules of who one is legally allowed to marry. I have countless cousins-in-law, several second cousins-in-law, nieces- and nephews-in-law and will very soon have a great nephew-in-law, all of whom would be most surprised to be referred to in that way.

DUDEN kennt "Schwiegeronkel" gar nicht; DWDS kennt nur ein einziges Beispiel (aus der ZEIT) mit "eine Art Schwiegeronkel".

#1: Personally, I would use a genitive form and say my wife's uncle.

Ich hätte in so einem Fall gesagt: Der Bruder von der Schwiegermutter bzw. vom Schwiegervater. Wie auch immer, den Begriff Schwiegeronkel kenne ich nicht.

Es ist vielleicht regional, denn wikipedia listet ihn auf:
http://de.wikipedia.org/wiki/Verwandtschaftsb...