From Latin 'lacus' and PIE *lokus (body of water) — same word as Scottish 'loch,' reunited in the British Isles.
A large body of water surrounded by land.
From Old English 'lacu' (stream, pool, lake), from Proto-Germanic *lakō (body of water), from PIE *leƵ- (to drip, to trickle) or *laku- (water, lake). The Proto-Germanic root is also seen in Old Norse 'lækr' (brook, stream) and Dutch 'laak' (pool, ditch). Independently, Latin 'lacus' (lake, basin, tank) — which has its own PIE root *laku- — reinforced the word in post-
English 'lake,' Scottish 'loch,' and Irish 'lough' are all the same PIE word *lókus (body of water) — inherited three times through three different language branches: Latin (→ French → English 'lake'), Goidelic Celtic (→ Scottish 'loch'), and Irish Celtic (→ Irish English 'lough'). Three spellings, three pronunciations, one 6,000-year-old word.