A huevo is a very common Mexican Spanish slang phrase that literally translates to "to the egg." However, its actual meaning is closer to "hell yeah," "damn right," or "by force/no matter what."
In the context of this song, it's used repeatedly to emphasize certainty and intensity, often in a somewhat aggressive or defiant way. For example, "a huevo la cagas" implies "you definitely screw up" or "you screw up no matter what." It's a raw and expressive phrase that adds a lot of character to the lyrics.
El Tri's "A Guevo La Cagas" is a hilariously blunt take on the consequences of getting too drunk. The title itself, roughly translating to "You really screw up," sets the tone perfectly. The song repeatedly points out how, when you get wasted, you end up making a fool of yourself.
It's all about those regrettable moments: "cuando te pones muy idiota" [when you get really idiotic] and then you're left with a messed-up face or situation. The lyrics mention fighting with your partner and then wanting to make up, or waking up with a headache and no money. It's a fun, no-holds-barred look at the messy side of overindulgence, with El Tri's signature rock 'n' roll attitude.