'leak' originates from Old English and Old Norse influences; Old English 'leccan' (or related forms) and Old Norse 'leka' meaning 'to let fall, to flow out'.

'leak' appeared in Middle English in forms such as 'leken'/'leke' and eventually became the modern English word 'leak', from which the adjective 'leaky' and adverb 'leakily' were formed.

Initially, the root words referred broadly to flowing out or letting fall; over time this narrowed to the modern sense of an unintended escape of liquid, gas, or information, and derivatives like 'leaky'/'leakily' describe that condition or manner.