loch (n.) Look up loch at Dictionary.com
late 14c., from Gaelic loch "lake, lake-like body," including the narrow, nearly land-locked arms of the sea found in the glacier-scoured landscape of west Scotland; cognate with Old Irish loch "body of water, lake," Breton lagen, Anglo-Irish lough, Latin lacus (see lake (n.1)). "The word was adopted in ONorthumbrian as luh" [OED]. The diminutive form is lochan.

The Loch Ness monster is first attested 1933; the loch is named for the river Ness that flows out of it at Inverness; the river name is probably from an Old Celtic word meaning "roaring one."