Because it depends on economy of expression, sign language is often far from subtle. Many evolve by matching terms and expressions with simple visual indicators tied to popularly held beliefs.
Like the fact that Americans are nuts.
According to an article by the South China Morning Post, Cantonese sign language, used across Southern China and Hong Kong, refers to nationalities this way:
Britons “A stroke of the nose to indicate its large size; this, in turn, is a reference to the Cantonese dai bei — “big nose” — which connotes perceived arrogance.”
Americans “A forefinger pointed towards the temple and twirled rapidly to indicate craziness.”
Japanese “A foul, two-handed universal gesture that intimates copulation. Whether this refers to Japanese people’s (reputedly) highly sexed nature or simply reflects age-old Chinese contempt for them is open to debate.”
So now you know.