"Pennis" doesn't appear in my Latin dictionaries. "Penna" or "pinna" means
"feather", but I don't see how "pennis" could be derived from "penna". If
Dall intended "pennis" as a variant of "penis", then you should know that
the word meant "tail" to the Romans, and the extension of its meaning to
the "male member" is apparently another fossilized joke or euphemism. Does
our resident lexicographer wish to <ahem> rise to the occasion by
elucidating this mystery?
 
Incidentally, pinna "feather" is also the source for "pen"; quill pens were
once made from large feathers. The large bivalve Pinna "pen shell" was
named by the Romans; Cicero used the word.
 
Andrew K. Rindsberg
Geological Survey of Alabama