Bough

Bough \bou\ , noun;
1. A branch of a tree, especially one of the larger or main branches

Deck the halls with boughs of holly...

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...When the bough breaks, the cradle will fall...

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I don't know about you, but if it weren't for these two songs I would probably have no idea what a bough was. But, its existence in these two old songs suggests the word's age. The origin of bough is Old English bog ("shoulder, arm," later "twig, branch") from Proto-Germanic *bogaz, which derived from Proto-Indo-European *bhagus ("elbow, forearm").

There are related words in other Proto-Indo-European language, including Sanskrit, Armenian, and Greek, but English is the only language where the definition has anything to do with trees.