prime minister

late 14c., "first, original, first in order of time," from Old French prime and directly from Latin primus "first, the first, first part," figuratively "chief, principal; excellent, distinguished, noble" (source also of Italian and Spanish primo), from Proto-Italic *prismos, superlative of PIE *preis- "before," from root *per- (1) "forward," hence "in front of, before, first, chief."

The meaning "of fine quality, of the first excellence" is from c. 1400. The meaning "first in rank, degree, or importance" is from 1610s in English. Arithmetical sense (as in prime number, one indivisible without a remainder except by 1) is from 1560s; prime meridian "the meridian of the earth from which longitude is measured, that of Greenwich, England," is from 1878. Prime time originally (c. 1500) meant "spring time;" the broadcasting sense of "peak tuning-in period" is attested by 1961.

c. 1300, "man consecrated to service in the Christian Church, an ecclesiastic;" also "an agent acting for a superior, one who acts upon the authority of another," from Old French menistre "servant, valet, member of a household staff, administrator, musician, minstrel" (12c.) and directly from Latin minister (genitive ministri) "inferior, servant, priest's assistant" (in Medieval Latin, "priest"), from minus, minor "less," hence "subordinate" (from PIE root *mei- (2) "small") + comparative suffix *-teros. Formed on the model of magister (see master (n.)).

Minister views a man as serving a church; pastor views him as caring for a church as a shepherd cares for sheep; clergyman views him as belonging to a certain class; divine is properly one learned in theology, a theologian; parson, formerly a respectful designation, is now little better than a jocular name for a clergyman; priest regards a man as appointed to offer sacrifice. [Century Dictionary, 1895]

The political sense of "high officer of the state, person appointed by a sovereign or chief magistrate of a country as the responsible head of a department of the government" is attested from 1620s, from notion of "one who renders official service service to the crown." From 1709 as "a diplomatic representative of a country abroad." A minister without portfolio (1841, in a French context) has cabinet status but is not in charge of a specific department.