One of the mysteries of the English language finally explained.
Definition of synecdochism in US English:
synecdochism
noun
1Grammar and Rhetoric. Synecdochic style; the use of synecdoche.
2Cultural Anthropology. A belief, or practice based on this belief, that a part of an object or person is equivalent to the whole, so that anything done to, or by means of, the part is held to take effect upon, or have the effect of, the whole.
Origin
Mid 19th century; earliest use found in Charles Badham (1805–1857), naturalist. From synecdoche + -ism.