'sap' originates from Old English, specifically the word 'sæp', meaning 'juice' or 'sap' in a plant.

'sap' appeared in Old English as 'sæp', continued into Middle English as 'sap', and became the modern English 'sap'. The verb sense ('to drain' or 'weaken') developed from the noun sense (plant juice) by figurative extension.

Initially, it meant 'plant juice' or 'vegetable juice'; over time it also came to mean 'vigor' or 'life force' and, by extension, a 'fool' (one lacking vigor or sense). The verb meanings ('to draw off sap', 'to drain strength', 'to undermine') developed from the noun sense.