I took a very pleasant tour through w3c, google and wikipedia and finally found the answer: an annotated XML spec where we find an excerpt of an email from the inventor of the name, James Clark, an Email from chairman Jon Bosak who suggested to use the
X letter, some other ideas for names and the final votes:
Votes Acronym Full Name
5 XML Extensible Markup Language
4 MAGMA Minimal Architecture for Generalized Markup Applications
3 SLIM Structured Language for Internet Markup
1 MGML Minimal Generalized Markup Language
I think, Jon Bosaks reply to James Clarks suggestion, to name it "Extensible Markup Language" gave birth to the acronym:
In my opinion, the U-combinations won't fly, but if we allow "X" to stand for "extensible", then I could live with (and even come to love) XML as an acronym for "extensible markup language", and I hereby now throw it into the list of current proposals.
Some bonus - from some old reports of the XML Special Interest Group, that I found while looking for some original quote that could answer the question:
M.15 Should the spec refer to XML as "The Extensible Markup Language" or as "Extensible Markup Language" without a definite article (e.g. in the first sentence)?
The WG elected to give no guidance to the editors on this issue (in the full expectation that the result would depend on which editor touched the file last).
Rationale: after several minutes' discussion and increasing hilarity, no consensus had been reached, but the end of the allotted time for the conference call had.