chastity
Explanation
The Joke
A woman with glasses sees a bearded man wearing socks with sandals and remarks: "Wow! I didn't know they made chastity belts for feet." The man is confused and asks: "For feet? Chastity... what are you talking about?" Then a moment of realization hits him -- shown across two panels where he simply says "Oh." and then "Oh." again -- as the full weight of the insult sinks in. The final panel shows his full outfit: socks with sandals, confirming the fashion crime in question.
The joke is that socks worn with sandals are so universally regarded as unattractive that the woman compares them to a chastity belt -- a medieval device designed to prevent sexual activity. In other words, wearing socks with sandals is such a turn-off that they function as a form of involuntary celibacy for one's feet (and by extension, for the person wearing them).
The Humor
The humor lies in the layered insult. The man does not immediately understand the joke, which makes it even funnier -- he has to work through the logic himself, arriving at the devastating conclusion in stages. The two successive "Oh." panels perfectly capture the slow dawning of a brutal realization. The comic plays on the widely shared cultural joke that socks with sandals are the ultimate fashion faux pas, elevating it from a simple style critique to an elaborate metaphor about romantic undesirability.