private (adj.)

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.

private (n.)

1590s, "private citizen," short for private person "individual not involved in government" (early 15c.), or from Latin privatus "man in private life," noun use of the adjective; 1781 in the military sense, short for Private soldier "one below the rank of a non-commissioned officer" (1570s), from private (adj.).

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Definitions of private from WordNet
private (adj.)
concerning one person exclusively;
each room has a private bath
Synonyms: individual
private (adj.)
confined to particular persons or groups or providing privacy;
private discussions
private property
a private place
private lessons
a private club
a private secretary
the former President is now a private citizen
public figures struggle to maintain a private life
private (adj.)
concerning things deeply private and personal;
private family matters
Synonyms: intimate
private (adj.)
not expressed;
secret (or private) thoughts
Synonyms: secret
private (n.)
an enlisted man of the lowest rank in the Army or Marines;
our prisoner was just a private and knew nothing of value
Synonyms: buck private / common soldier