The act of going out to the bar or nightclub and finding the biggest, fattest, ugliest girl possible; then taking her back to your place and having sex with her.
Usually done by groups of men as a form of competition to see who can get the biggest girl. As proof, the curtains in the persons bedroom/hotel room are usually cracked so others can look in. If you're ballsy and shameless, you throw the curtains wide open.
Rick definitely won at Harpooning a Whale. That girl looked like an upside down mushroom from a block away.