Meanings

underside of a bird's wing

greater palace

urasuji

Astralium haematragum (species of turban shell)

backdoor admission to a school

underworld

inside the back cover (book)

act of treachery

Japanese mountain ash (Sorbus japonica)

sole (of a shoe)

underground website (often critical or containing unofficial information)

underground website or online message board for a school where students typically exchange information on tests, commit cyberbullying, etc.

suede

back stairs

lingual braces (dentistry)

to make a mistake (e.g. in choosing one's melds)

peaceable

the reverse (side)

secret accounts

arena (e.g. international arena)