The dictionary I use is one based on Buddhadatta's concise dict., and it
gives the following translations:
pakkhiya: belonging to a faction; siding with; fortnightly.
pakkhii: a bird; the winged one.
pakkha: a cripple; a lamp person; side; party; faction; side of the body; a
flank, a wing; a fortnight. (adj.) adherent; associated with.
Curiously, the Anagata-bhayani Suttas sent by Dimitry where translated
by the same Thanissaro Bhikkhu who translated bodhi-pakkhiya-dhamma as Wings
to Awakening. Again in the AN IX.1 available at ATI 'sambodhipakkhikaana.m
dhammaana.m' appears as "the wings to self-awakening".
The messages posted about this subject made me think that both
renderings are acceptable depending on the context where they're used. It
sounds well in Thanissaro's book title and there is no problem with such
usage since this choice did not obscure the meaning of the expression in the
book. On the sutta refered to above, however, I think that the choice was
not so appropriate.
It seems that when the Buddha used this expression he didn't mean
anything related to "wings", but it's even possible that he constructed the
phrase this way to give it a sort of "double meaning", likewise the ariyan
custom of worshipping the six quarters as father, mother, etc.