from
The Collaborative International Dictionary of English v.0.48
Incest \In"cest\, n. [F. inceste, L. incestum unchastity,
incest, fr. incestus unchaste; pref. in- not + castus chaste.
See {Chaste}.]
The crime of cohabitation or sexual intercourse between
persons related within the degrees wherein marriage is
prohibited by law; broadly, sexual relations between closely
related persons, usually taken as first cousins or closer.
--Shak.
[1913 Webster +PJC]
{Spiritual incest}. (Eccl. Law)
(a) The crime of cohabitation committed between persons who
have a spiritual alliance by means of baptism or
confirmation.
(b) The act of a vicar, or other beneficiary, who holds two
benefices, the one depending on the collation of the
other.
[1913 Webster]