Cui malo? - Who suffers a detriment?
Cui peccare licet peccat minus - One who is allowed to sin, sins less. (Ovid)
Cuius regio, eius religio - He who rules, his religion

Cuiusvis hominis est errare; nullius nisi insipientis in errore perseverare

Any man can make a mistake; only a fool keeps making the same one

Cuivis dolori remedium est patientia - Patience is the cure for all suffer
Culpa - A sin
Culpam poena premit comes - Punishment closely follows crime as its companion. (Horace)
Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt - When catapults are outlawed, only outlaws will have catapults
Cum grano salis - With a grain of salt. (Pliny the Elder?)
Cum homine de cane debeo congredi - Excuse me. I've got to see a man about a dog