Transa is a fascinating word because it has multiple meanings, making it quite versatile and often used in colloquial Brazilian Portuguese. While it can mean a 'transaction' or 'deal', it also commonly refers to a 'fling' or 'affair', or even a 'vibe' or 'energy'.
In Caetano Veloso's lyrics, "A barca da transa chegou" (The boat of transa arrived), the word transa is used in a very open-ended way. It evokes a sense of something exciting, perhaps illicit, or simply a powerful, transformative energy that has arrived. This ambiguity makes it intriguing and memorable, inviting listeners to ponder its deeper meaning within the song's context of a boat that "transou, nem parou" (made out, didn't stop) and is heading to "mares de loucura" (seas of madness).
The song turns a boat into a stand-in for restless desire, or maybe for the country itself. It opens on an arrival that never settles: "A barca da transa chegou" (The boat of the deal/hookup has arrived), "a barca pintou e bordou" (the boat painted and embroidered itself, i.e. dressed up, put on a show), "a barca transou, nem parou" (the boat hooked up, didn't even stop), "já vai partir" (it's already leaving) — the whole chorus is one continuous motion, arriving and departing in the same breath.
The verses widen the boat's journey into something closer to a warning: "Por outros mares de loucura, vai" (Through other seas of madness, it goes), "ela fatura e sai" (it cashes in and leaves), "e nunca vai chegar" (and it will never arrive) — the profit-taking has no destination. "Sem praia segura no mar dessa multidão" (With no safe shore in the sea of that crowd), "a barca procura em vão" (the boat searches in vain), "a barca não pode parar" (the boat can't stop) — search and motion with no landing, dressed as a party song but built entirely out of things that never resolve.