lady-stuff
Explanation
The Joke
A man knocks on the bathroom door and asks, "Why do you lock the bathroom door? We've been together for years and have I ever barged in there?" From behind the door, the woman (clearly annoyed) says, "Ugh." She then calls out, "I'm having lady stuff going on in here!" The man immediately backs off: "Nevermind!"
The final panel reveals what is actually happening behind the locked door: the woman is sitting on the floor conducting what appears to be a dark ritual or bizarre hobby -- she has a small animal (possibly a opossum or cat) and what looks like a bowl of something sinister, with the caption reading something like "I hate lying to my love, but #cosmocalyx" and then showing a scene of "unraveling the universe, as is my right." The joke is that "lady stuff" is not menstruation or any conventional feminine hygiene activity -- it is something genuinely strange and inexplicable that the woman hides behind the socially unquestionable shield of "lady stuff," knowing her partner will never press for details.
The Humor
The comedy hinges on the social dynamic where mentioning "lady stuff" (implying menstruation or other female-specific bodily functions) is such an effective conversation-stopper for men that it can be used to conceal literally anything. The man's immediate "Nevermind!" demonstrates how powerfully the phrase shuts down all further inquiry. The reveal of what she is actually doing -- something far stranger than anything biological -- rewards the reader's curiosity while satirizing both male squeamishness about women's bodies and the way that squeamishness can be weaponized as a privacy tool.