/in"sest/ , n.
1. sexual intercourse between closely related persons.
2. the crime of sexual intercourse, cohabitation, or marriage between persons within the degrees of consanguinity or affinity wherein marriage is legally forbidden.
[ 1175-1225; ME incestus (n.) sexual impurity, deriv. of incestus (adj.) profane, sexually impure ( in- IN- 3 + -cestus comb. form of castus CHASTE), by analogy with v. nouns derived with -tus ]