A woman spreads lye over her arms and legs every day. Lye is a harsh chemical that burns skin, yet she keeps doing it, and the poem tells us why in her own words: "I'm gonna look so good." The whole poem turns on the gap between that cheerful thought and the pain around it.
The speaker is hard to pin down. She feels burning and itching, but what she's thinking is hopeful, even upbeat. The casual "gonna" makes her sound breezy, almost like someone getting ready for a night out. That tone makes the scene more unsettling, because she treats harm as a normal part of a routine.
The title, "Cosmetinaut," seems to blend "cosmetic" with "astronaut," as if she's an explorer braving a dangerous place for the sake of appearance. The poem's mood is darkly funny and uneasy. It seems to poke at how far people will go, and how much pain they'll tolerate, to meet ideas of beauty.