(3 syl.). To many Roman laws this tag was appended, “If any one breaks this law, sacer esto, ” i.e. let his body, his family, and his goods be consecrated to the gods. When a man was declared sacer, anyone might kill him with impunity. Anyone who hurt a tribune was held a sacer to the goddess Ceres. Ex in this word is intensitive.
“If anyone hurt a tribune in word or deed, he was held accursed [sacer], and his goods were confiscated.” —Livy, iii. 55; see also Dionysius, vi. 89, and viii. 17.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894