Bar
Explanation
The Joke
A man is enthusiastically explaining bar notation to someone. He says that a bar (the line over a digit) is basically the digit divided by 2, and that you can put a line through any constant -- there is t-bar, z-bar, g-bar, and so on. His listener responds with "Yeah!"
The man then continues: "I don't say 'one', I don't call anyone 't-bar', I don't call people names, I just call them 't-bar-wits.'" He then adds, "You don't call anyone anything because people don't like being around you." In the final panel caption, the text reads "All but imagining it, they say."
The Humor
The comedy arises from the classic SMBC formula of taking a dry academic topic -- mathematical bar notation -- and running it into absurd social territory. The character starts with a seemingly genuine explanation of how overbars work in mathematics and physics (where a bar over a symbol often denotes a mean, conjugate, or specific variant of a quantity), but then pivots to using this notation system as a way to rename everything in daily life, including insults ("t-bar-wits" as a play on "nitwits" or "halfwits"). The punchline deflates the character entirely by revealing he is alone and friendless, precisely because of this kind of insufferable pedantic behavior. It is a joke about the social cost of being excessively nerdy.
References
Bar notation is used across mathematics and physics. An overbar on a variable commonly represents a mean (x-bar for the sample mean), a complex conjugate, or a complement in set theory. In particle physics, placing a bar over a particle symbol denotes its antiparticle. The comic plays on the universality of this notation convention.