1825, from Modern Latin Petunia (1789), from French petun (16c.), an obsolete word for "tobacco plant," from Portuguese petum, from Guarani (Paraguay) pety. It has a botanical affinity to the tobacco plant. The word first is recorded (in German) as bittin; it survives as the regular word for tobacco only in Breton butun, but it was in use in English in 17c.
Many haue giuen it the name, Petum, whiche is in deede the proper name of the Hearbe, as they whiche haue traueiled that countrey can tell. [John Frampton, translation of Nicolás Monardes' "Joyful Newes Oute of the Newe Founde Worlde," 1577]