Unforgivable! (ゆるさなぁーい！ Yurusanāi!, lit. "Forgive Me!") is a Self Destruction technique used by Cell in his Semi-Perfect form. It is unknown if Cell is capable of using this in his Imperfect or Perfect forms, though it is likely.
When performed, Cell pumps himself up like a balloon and explodes after one minute. The resultant explosion is capable of destroying an entire planet. If Cell is attacked in any way before the countdown reaches zero, he will explode immediately. Once activated, according to Cell, he cannot reverse the technique.
After losing Android 18 and reverting to his Semi-Perfect form, Cell uses Unforgivable! in an attempt to blow up the Earth to defeat Super Saiyan 2 Gohan during the Cell Games. However, Goku prevents him from doing so by taking Cell to King Kai's Planet via Instant Transmission as he decides to make the ultimate sacrifice in order to save the Earth, and Cell ends up destroying this planet instead, killing Goku, King Kai, Bubbles, and Gregory. It also presumably kills the absorbed Android 17. A small part of Cell survives the explosion, and he regenerates and becomes even stronger than before, reaching his Super Perfect form.
Appearances in games
Unforgivable! was named in the video game Dragon Ball Z: Budokai Tenkaichi 3; in the game, he rushes at his opponent, then says "I won't allow this, YOU WON'T GET AWAY!", pumps up, and self-destructs, damaging himself and his opponent. The technique is called Self-Destruct in Dragon Ball Z: Supersonic Warriors 2. Though the move itself never appears in the game Dragon Ball Xenoverse, Cell almost uses it to try to get a reaction out of the Future Warrior after they complete one of his training missions (implying that the technique can be used in his Perfect Form, though this may have been a bluff given that Cell was not actually serious).