The motif recurring in folklore (legends, jokes) of an overly large (even grotesquely vast) vagina--in which objects become lost, into which entire persons tumble, etc. Clearly a projection of masculine fears of sexual inadequacy, the image epitomizes the concept of the voracious and insatiable sexual appetite attributed to some (or all) women. The term was coined (in English, at least) by the folklorist Elissa R. Henken; see Henken and Mariamne Whatley, Did You Hear about the Girl Who--?: Contemporary Legends, Folklore, and Human Sexuality (NY: New York UP, 2000) 121.
Joke (based on the motif of the vagina infinita): A woman is being degitally pleasured. "Oh, your ring hurt me!" "That's not my ring, it's my watch." Legend/rumor (exemplifying the motif of the vagina infinita): A woman loses a tampon (or a hotdog) inside herself.