E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
All the sea which is not the property of a particular country. The sea three miles out belongs to the adjacent coast, and is called mare clausum. High-seas, like high-ways, means for the public use. In both cases the word high means chief, principal. (Latin, allum, the main sea; allus, high.)