The M4 Medium SHERMAN tank.
Might as well start with this wiki page
Took the addition of the British 17 pdr gun to make it a really effective AFV now called the Firefly, although it was still under armoured compared to the concurrent generation of German tanks. (Pather, Tiger 1e, Tiger II)
American crews were told they were driving the best tank in the world - in reality they were given a nasty attrition unit with a puffy low velocity 75mm popgun. (see Kelly's Heroes for an idea just how close a Sherman had to get to inconvenience a Tiger). Even the Sherman's much vaunted maneuverability was not much better than the Tiger 1e (a 56 tonne monster with an 88mm L56 gun).
Rhyming slang for an American (Yank).
Also rhyming slang term for a Jodrell Bank or wank.