A first-generation Big Block Chevrolet V8 engine. It was in the middle of the three "W-head" engines offered by Chevrolet; the others being the 348 and (very rare W-head) 427. It was phased out by the Mark IV series "Turbo Jet" big blocks by 1965.
The 409-inch engine was first available in 1961, and dominated the drag strip and stock car circuits in the early 60s. In 1963, the Impala SS 409 produced around 425 horsepower; an impressive feat for the technology of that era.
The Chevrolet 409 is subject matter of the Beach Boys song of the same name.