A fork of Rural Dictionary
This can refer to "one who makes a hobby of exploring and studying caves" such as the original definition suggests, but "caves" can have more than one meaning. One such would be the cave representing a woman's genitals, where the "spelunking" is done by male genitals. The context still stands fairly clear, with exploring being the main activity- although, "studying" would be changed to something more along the lines of "pounding as hard as he can".
Example 1: "I want to go down on her and do a little spelunky!" Example 2: Guy 1: "Did you smash her last night?" Guy 2: "I didn't just smash her- I SPELUNKY'D that PUSSY"