I'm looking for a word that has had its meaning influenced by a US
Supreme Court Justice.  This word now implies something more lewd or
pejorative than it had prior to its usage by the said Justice. 
Apparently some aspect of the case and the way it was used caused some
confusion that eventually become a colloquialism.    My friend thinks
it starts with a P, but who knows.

Your friend may be referring to Supreme Court Justice Potter Stewart's
famous famous "definition" of hard-core pornography, "I know it when I
see it."

Here is his one-paragraph concurring opinion in that case (the quote
is near the end):

"It is possible to read the Court's opinion in Roth v. United States
and Alberts v. California, 354 U.S. 476 , in a variety of ways. In
saying this, I imply no criticism of the Court, which in those cases
was faced with the task of trying to define what may be indefinable. I
have reached the conclusion, which I think is confirmed at least by
negative implication in the Court's decisions since Roth and Alberts,
1 that under the First and Fourteenth Amendments criminal laws in this
area are constitutionally limited to hard-core pornography. 2 I shall
not today attempt further to define the kinds of material I understand
to be embraced within that shorthand description; and perhaps I could
never succeed in intelligibly doing so. But I know it when I see it,
and the motion picture involved in this case is not that."

Jacobellis v. Ohio, 378 U.S. 184 (1964) 
http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&vol=378&invol=184