Tail
Explanation
The Joke
The comic shows a bespectacled man (drawn in Weinersmith's typical "nerd" style) excitedly holding up a calculator and shouting "Yeah! Shake that fat tail, baby!" Below the panel, the caption reads: "This is how statisticians celebrate rare events."
The joke is a play on the phrase "fat tail." In everyday parlance, "shake that fat tail, baby" sounds like something you would shout at a dance club -- a suggestive compliment about someone's body. But in statistics, a "fat tail" (or "heavy tail") refers to a probability distribution where extreme, rare events occur more frequently than a normal (Gaussian) distribution would predict. The statistician is not catcalling anyone; he is excitedly celebrating that a rare, improbable event has occurred, as indicated by the fat-tailed distribution on his calculator.
The Humor
The comedy works through the double meaning of "fat tail." The image of an excited statistician shouting what sounds like a crude pickup line -- but is actually a technical term -- is inherently funny because it plays on the contrast between the buttoned-up world of statistics and the raucous atmosphere of a nightclub. The visual of the nerdy man with his calculator held aloft like a party-goer with a drink adds to the absurdity. It is a classic "nerd joke" that rewards the reader for knowing the statistical meaning of the term.
References
In probability theory and statistics, "fat-tailed distributions" (also called heavy-tailed distributions) are probability distributions that have tails heavier than an exponential distribution. Examples include the Cauchy distribution, the Pareto distribution, and the Student's t-distribution. Fat tails are important in finance (where they explain market crashes and other extreme events), natural disaster modeling, and many other fields. The concept was popularized by Nassim Nicholas Taleb in his book "The Black Swan" (2007), which argued that rare, fat-tail events have an outsized impact on the world.