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.