But as of 2005, the FAA *has* published an explicit definition of the term,
and has done so in the AIM, a publication that is intended in part to
explain and clarify to pilots some details of the regulations (even though
the AIM's contents do not themselves *constitute* regulations).

.... and thus do not constitute legal definitions either.

Let's ask a different question. Suppose it comes to the FAA's attention
that a pilot flew undeiced into conditions which no longer constitute
"known icing" under the new AIM definition (but once did), and the FAA
decided to pursue legal action against the pilot on the basis of their
old definition and case law. Since the FAA gets to interpret the rules
their way, suppose the FAA even wins the case. What recourse does the
AIM give the pilot, which would be recognized by appropriate judicial
people to get the pilot off the hook?

Especially given their applictaion of "careless and reckless" to actions
which are =specifically= legal (IFR flight in IMC in uncontrolled airspace).

So far, though, nothing
you've said gives me any reason to think the FAA could or would prosecute a
pilot for abiding by the AIM: there is no precedent for their trying to do
that, no explanation of how it could survive judicial review, and no
statement by the FAA denying their intention to stand by the definitions
they currently publish in the AIM.

They have prosecuted pilots for abiding by the FARs, why not the AIM.
The case I refer to (iirc) involved a pilot who took off from an
uncontrolled field in uncontrolled airspace; after making his own
determination that it would be safe, he took off IFR and was busted as
"careless or reckless".

Jose
--
Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.