Resacón is a colloquial and more intense version of resaca, meaning "hangover". The suffix -ón in Spanish often implies a larger or more significant version of something, so resacón conveys a "massive" or "tremendous" hangover.
In the song, the narrator wakes up with extreme confusion and pain, leading to the line, "No sé si podré soportar este tremendo resacón" (I don't know if I can handle this tremendous hangover). It's a vivid and relatable word that captures the morning-after struggle described in the lyrics, making it memorable and engaging.
Ever woken up after a wild night and felt like the world was literally upside down? That's exactly where Estopa takes us in "Del Revés"! Our singer wakes up in his room with a massive headache and total confusion, not remembering anything from yesterday. He's so out of it that he can't even face breakfast, saying, "No bajar a desayunar, tostaditas en la fondue" [Not going down for breakfast, toast in the fondue].
He's trying to figure out if the crazy night actually happened or if it was all just a "experiencia paranormal / Que soñé y se me puso el mundo al revés" [paranormal experience / That I dreamed and my world was turned upside down]. Despite the hangover, he admits he had a phenomenal time! Eventually, he decides to leave his room, bracing himself for the "tremendo resacón" [tremendous hangover]. It's a fun, relatable look at those mornings where everything feels a bit... del revés!