A vagina-specifically one with "spongy" or thick lips. The term comes from Corpus Spongiosum, which is the name for the paired sponge-like caverns on either side of the penis that engorge with blood upon sexual arousal. "Spoonjahns" is the feminine analog for that male structure. I believe that this may also be the origin of the even shorter "poon".
You should have seen the spoonjahns on that chick at the go-go bar!