'weir' originates from Old English, specifically the word 'wer', where the root meant 'an enclosure or fence (used to trap fish)'.

'weir' changed from Old English 'wer' into Middle English forms such as 'were'/'weir' and eventually became the modern English word 'weir'.

Initially it meant 'an enclosure or fish-trap', but over time it evolved to include the current meaning of 'a barrier across a river to control water flow and catch fish'.