sap [OE] Old English sæp ‘vital fluid’ is probably of Germanic origin. The verb (as in sapped his energy) dating from the mid 18th century is often interpreted as a figurative use of the notion ‘drain the sap from’ but is unrelated. It comes originally from the late 16th-century verb sap ‘dig a tunnel or covered trench’ thus meaning ‘undermine’. The latter is from French saper, from Italian zappa ‘spade, spadework’, probably from Arabic sarab ‘underground passage’, or sabora ‘probe a wound, explore’. This is where the military engineers called sappers [E17] get their name.