plantain (n.1)
"banana," 1550s, plantan, from Spanish plátano, plántano, probably from Carib palatana "banana" (Arawak pratane), and altered by association with Spanish plátano "plane tree," from Medieval Latin plantanus "plane tree," itself altered (by association with Latin planta "plant") from Latin platanus (see plane (n.4)). So called from the shape of its leaves. There is no similarity or relation between this plant and plantain (n.2).
plantain (n.2)
"weed of the genus Plantago," mid-13c., from Anglo-French plaunteyne, Old French plantain, from Latin plantaginem (nominative plantago), the common weed, from planta "sole of the foot" (see plant (n.)); so called from its flat leaves.