late 14c., "pertaining or belonging to oneself, not shared, individual; not open to the public;" of a religious rule, "not shared by Christians generally, distinctive; from Latin privatus "set apart, belonging to oneself (not to the state), peculiar, personal," used in contrast to publicus, communis; past participle of privare "to separate, deprive," from privus "one's own, individual," from Proto-Italic *prei-wo- "separate, individual," from PIE *prai-, *prei- "in front of, before," from root *per- (1) "forward." The semantic shift would be from "being in front" to "being separate."
Old English in this sense had syndrig. Private grew popular 17c. as an alternative to common (adj.), which had overtones of condescension. Of persons, "not holding public office," recorded from early 15c.
In private "privily" is from 1580s. Related: Privately. Private school is from 1650s. Private parts "the pudenda" is from 1785. Private enterprise first recorded 1797; private property by 1680s; private sector is from 1948. Private eye "private detective, person engaged unofficially in obtaining secret information for or guarding the private interests of those who employ him" is recorded from 1938, American English.