feckful
Explanation
This comic plays on the word "feckless," which in modern English means lacking initiative, irresponsible, or worthless. One character asks a perfectly reasonable linguistic question: how can someone be "feckless" if they can't also have "feck"?
The other character explains that "feck" is an obsolete term that probably comes from "effect," used in the sense of having impact or substance. So "feckless" originally meant "without effect" or "ineffective," and "feckful" would mean effective, vigorous, or capable. As the character notes, if you go around saying "feckful," people will just think you're using a euphemism for a certain profanity (since "feck" sounds very close to a well-known expletive, and is in fact used as a minced oath in Irish English).
The punchline is that this etymological discovery immediately gets weaponized for activism. Armed with the knowledge that "feck" is a real, legitimate English word, the characters create a protest sign reading "TOGETHER WE CAN FECK THE WORLD" -- a slogan that is technically wholesome (meaning "together we can make a real impact on the world") but reads as hilariously vulgar to anyone who doesn't know the word's actual meaning.
The humor lies in the gap between linguistic correctness and social perception. The comic celebrates the joy of finding loopholes in language -- using a perfectly proper, etymologically sound word that nonetheless sounds utterly obscene.