1. [n]  the keystone of an arch
2. [n]  expandable metal or wooden wedge used by printers to lock up a form within a chase

COIGNE is a 6 letter word that starts with C.

1. \Coigne\ (koin), n. [See {Coin}, n.]
   A quoin.
   
         See you yound coigne of the Capitol? yon corner stone?
                                                  --Shak.
   
   

2. \Coigne\, Coigny \Coign"y\, n.
   The practice of quartering one's self as landlord on a
   tenant; a quartering of one's self on anybody. [Ireland]
   --Spenser.