c.1330, "body of men associated by common interest," from O.Fr.
fraternité, from L.
fraternitatem (nom.
fraternitas), from
fraternus "brotherly," from
frater "brother," from PIE
*bhrater (see
brother). College Greek-letter organization sense is from 1777, first in reference to Phi Beta Kappa; shortened form
frat first recorded 1895.
Fraternize is attested from 1611, "to sympathize as brothers;" sense of "cultivate friendship with enemy troops" is from 1897; used oddly by World War II armed forces to mean "have sex with women from enemy countries."
Fraternal is 1421, from M.L.
fraternalis, from L.
fraternus.