Aprontar is an interesting verb that goes beyond a simple translation of "to get ready." While it can mean to prepare, in a more colloquial and common usage, especially in Brazil, it often implies getting ready to cause trouble, to play pranks, or to stir up mischief.
In the context of the song, the line "Porque quando escurece, só estou a fim de aprontar" (Because when it gets dark, I'm only in the mood to stir up trouble) perfectly captures this mischievous connotation. It suggests a desire for nocturnal adventures and a break from the mundane, making it a captivating word for learners.
"Tédio (Com Um T Bem Grande P'ra Você)" is Legião Urbana doing nothing, on purpose, and making it sound like a statement. The verses list out a day with no plans — no gas, no car, sleeping through the afternoon — and the chorus turns that emptiness into a taunt: "Tédio com um T bem grande pra você" [Boredom with a capital T for you]. It's less "I'm bored" than "watch me be bored at you."
There's a shrug built into the writing. "Também não tenho nada de interessante pra fazer" [I also don't have anything interesting to do] isn't an apology, it's the whole plan. Then the closing lines flip the mood: once it gets dark, the narrator admits, "só estou a fim de aprontar" [I just want to cause trouble]. So the boredom was never really about having nothing to do — it's a pose the band wears right up until the night gets more interesting.