id
Explanation
A parent asks their daughter Sally if she cleaned her room. Sally replies "I idn't idn't," which confuses the parent. Sally (or another character) then explains: "'Idn't' is the imaginary negation. Use it twice and it changes to 'didn't.' Four times, it returns to 'did.'" The parent, trying to work through the logic, says "So... I didn't idn't idn't not," and asks "Is this a real thing or did you make it up?" Sally simply replies "I idn't!" -- leaving the parent (and reader) unsure whether that means she did or didn't.
The joke is a math/linguistics crossover that maps the concept of imaginary numbers onto grammar. In mathematics, the imaginary unit i has the property that i squared equals -1, so multiplying by i twice gives you a negation, and four times brings you back to the original. The comic applies this same cyclic property to negation in language: "idn't" is an imaginary negation that, when squared (used twice), becomes a real negation ("didn't"), and when used four times returns to the affirmative ("did").
The humor works on multiple levels. First, it is a clever mathematical analogy that rewards readers who understand complex numbers. Second, it functions as a parenting joke about children finding creative ways to give non-answers when asked if they did their chores. Sally's final "I idn't!" is the perfect evasion -- it is literally undecidable whether she cleaned her room, much like how the imaginary unit is neither positive nor negative. The comic pokes fun at how mathematical abstraction, taken out of its domain, can be used to obfuscate simple yes-or-no questions.