“Sex” is the Latin word for “six”

It has its roots in Proto-Indo-European “*swéḱs”, whence Proto-Germanic “*sehs” (thereby resulting in English “six”, German “sechs”, Dutch “zes”, Swedish “sex”, and Gothic “saihs” among others), Ancient Greek “ἕξ” (héks), Sanskrit “षष्” (ṣáṣ), Lithuanian “šeši”, Irish “sé”, Russian “шесть” (šest’), Armenian “վեց” (vecʿ), Albanian “gjashtë”, and Tocharian A “ṣäk”