The nipple spike is the name given to Captain Falcon's spike, performed by hitting the opponent with the highest out of the three hitboxes on his down aerial (whereas the other two hitboxes result in a meteor smash). Its discovery was posted on Smashboards in this thread started by Juce and was verified as a true spike shortly after.
Since its discovery as a true spike, the nipple spike has seen little to no widespread inception into the playstyles of Captain Falcons across the community. One possible cause of this could be because of how very difficult the move is to precisely connect with. This could also be paired with the assumption that another move, such as Knee smash, would make just as good of a finisher, if not a better one. While the nipple spike can be very hard to connect with, it is potentially very powerful as it has the ability to ensure low percent deaths in situations that may otherwise guarantee nothing in terms of death (such as is frequently the case on stages with wide blast lines, i.e. Dream Land).
However, it is also important to note the many launching moves available to Captain Falcon. Depending on the opponent's character and percentage, the nipple spike can be very easy to combo into. A post made in the nipple spike thread by Eighteenspikes documented rudimentary percentages at which a nipple spike could be done on a Fox, Falco, or Marth after a Raptor Boost or down aerial was used as a launcher. Since then, there has been no research done regarding the nipple spike.