a suffix used to form abstract nouns from adjectives with stems in -t, -te, -tic, and especially -nt (democracy; accuracy; expediency; stagnancy; lunacy), and sometimes used to form action nouns (vacancy; occupancy).
a suffix of nouns denoting rank or office, sometimes attached to the stem of a word rather than to the word itself: captaincy; magistracy.

Origin of -cy

representing French -cie, -tie, Latin -cia, -tia, Greek -kia, -keia, -tia, -teia; in most cases to be analyzed as consonant + -y3, the consonant making the whole or the last member of the preceding morpheme
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British Dictionary definitions for -cy



(forming nouns from adjectives ending in -t, -tic, -te, and -nt) indicating state, quality, or conditionplutocracy; lunacy; intimacy; infancy
(forming abstract nouns from other nouns) rank or officecaptaincy

Word Origin for -cy

via Old French from Latin -cia, -tia, Greek -kia, -tia, abstract noun suffixes
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012