in·cest 
(ĭnsĕst′)
(ĭnsĕst′)n.
1. Sexual relations between persons who are so closely related that their marriage is illegal or forbidden by custom.
2. The crime of sexual relations with a person defined by statute as too closely related.
[Middle English, from Latin incestum, from neuter of incestus, impure, unchaste : in-, not; see IN-1 + castus, pure, chaste; see kes- in the Appendix of Indo-European roots.]