From the Online Etymology dictionary:
in Cartographer Martin Waldseemüller’s treatise “Cosmographiae Introductio,” from Mod.L. Americanus, after Amerigo Vespucci (1454-1512) who made two trips to the New World as a navigator and claimed to have discovered it. His published works put forward the idea that it was a new continent, and he was first to call it Novus Mundus”New World.” Amerigo is more easily Latinized than Vespucci. The name Amerigo is Germanic, said to derive from Goth. Amalrich, lit. “work-ruler.” The O.E. form of the name has come down as surnames Emmerich, Emery, etc. The Italian fem. form merged into Amelia. Colloquial pronunciation “Ameri-kay,” not uncommon 19c., goes back to at least 1643 and a poem that rhymed the word with away. Amerika “U.S. society viewed as racist, fascist, oppressive, etc.” first attested 1969; the spelling is German, but may also suggest the KKK.