water moccasin

noun

the cottonmouth.

any of various similar but harmless snakes, as a water snake of the genus Nerodia.

Origin

1815-25, Americanism

Examples from the web for water moccasin

So, the next time you're sitting on a dock and a snake swims by, don't worry--it's not a water moccasin.
Many people falsely believe that any snake seen in or near the water is a water moccasin, also known as a cottonmouth.
It goes by the common names of cottonmouth or water moccasin.
Sometimes this snake is mistaken for the poisonous water moccasin snake.

British Dictionary definitions for water moccasin

noun

a large dark grey venomous snake, Agkistrodon piscivorus, of swamps in the southern US: family Crotalidae (pit vipers) Also called cottonmouth