Definitions
1.verbTo take part in tombstoning: to jump into the sea, etc. from a cliff or other high point so as to enter the water vertically straight.
2.verbFor a surfboard to stand upright half-submerged in the water (like a tombstone, above) because the surfer is underwater with his or her legrope pulled tight. Often this indicates a surfer in difficulty, either held down by the power of a wave or unconscious and unable to get to the surface.
3.verbTo replace (an object or data) with a tombstone marker.