Yes, I know that "cdecl" is the name of a prominent calling convention, so please don't explain calling conventions to me. What I'm asking is what the abbreviation (?) "cdecl" actually stands for. I think it's a poor naming choice, because at first sight it reminds one of "C declarator" (a rather unique syntactic aspect of C). In fact, there is a program called cdecl whose sole purpose is to decipher C declarators. But the C declarator syntax has absolutely nothing to do with calling conventions as far as I can tell.
Simplified version: "stdcall" stands for "standard calling convention". What does "cdecl" stand for?
The term CDECL originates from Microsoft's BASIC and their Mixed Language Programming ecosystem. The ecosystem permitted any of the Microsoft's four major languages (BASIC, FORTRAN, Pascal and C) to make calls to any other. Each language had a slightly different calling convention, and each had a way to declare an external function or procedure to be using a specific convention.
In BASIC, the DECLARE statement had to be used before you could call an external function with the CALL statement. To denote an external FORTRAN or Pascal procedure or function, you would write one of
DECLARE SUB Foo ()
DECLARE FUNCTION Foo ()
C calling conventions differed from the other languages in part because the arguments were pushed on the stack in reverse order. You would inform BASIC of this by adding the CDECL modifier:
DECLARE SUB Foo CDECL ()
DECLARE FUNCTION Foo CDECL ()
By contrast, when writing in FORTRAN or Pascal, the modifier is [C]. This is an indication CDECL was specific to BASIC's DECLARE statement and not a previously established term. At the time, there was no specific term for "C calling conventions". Only with the advent of new calling conventions in WIN32 (stdcall, fastcall, etc) did "cdecl" get co-opted and become the de-facto name to refer to the legacy conventions in the absence of another term.
In summary, CDECL means "C declaration". It had its origins in BASIC compilers, not C compilers, and it was an arbitrarily-chosen BASIC keyword, and somewhat redundant because plain "C" could not be a keyword.
Details about CDECL can be found in this 1987 document: