vagina

The Online Etymology Dictionary defines vagina (n.) as the "sexual passage of the female from the vulva to the uterus."  But that is a modern medical use of the word. The word in Latin was not used in an anatomical sense in classical times.

Originally the word meant a "sheath, scabbard, covering; sheath of an ear of grain, hull, husk" (plural vaginae), from wag-ina- (cognates: Lithuanian vožiu "ro cover with a hollow thing"), from root wag- "to break, split, bite."  This probably was most commonly thought of as a sheath made from a split piece of wood to hold and protect a knife or sword blade.

As this illustration shows, both the sheath opening's shape and the sexual connotation of the sword entering the sheath probably led to the more modern anatomical usage.

Sheath for a sword showing a shape similar to the anatomical vagina

In the 1680s, medical Latin began to use the term in the anatomical sense that we know it today.