sapyoudaoicibaDictYouDict[sap 词源字典]
sap: English has three distinct words sap. The oldest, ‘plant-juice’ [OE], goes back to a prehistoric Germanic *sappam, which also produced German saft ‘juice’. This in turn was a descendant of Indo-European *sapon-, from which came Latin sapa ‘new wine’. Sap ‘undermine’ [16] was borrowed via French saper from Italian zappare, which may have been ultimately of Arabic origin.

Its original literal sense ‘dig a trench or tunnel underneath in order to attack’ has now been largely superseded by the metaphorical ‘weaken’, which has been heavily influenced by sap ‘plant-juice’ (from the notion of ‘draining sap from a plant’). The colloquial sap ‘fool’ [19] may be short for an earlier sapskull, a compound formed from sap in the now seldom heard sense ‘sapwood’ – hence ‘wooden head’.

[sap etymology, sap origin, 英语词源]
sap (n.1)youdaoicibaDictYouDict
"liquid in a plant," Old English sæpm from Proto-Germanic *sapam (cognates: Middle Low German, Middle Dutch, Dutch sap, Old High German saf, German Saft "juice"), from PIE root *sab- "juice, fluid" (cognates: Sanskrit sabar- "sap, milk, nectar," Latin sapere "to taste," Irish sug, Russian soku "sap," Lithuanian sakas "tree-gum"). As a verb meaning "To drain the sap from," 1725.
sap (n.2)youdaoicibaDictYouDict
"simpleton," 1815, originally especially in Scottish and English schoolboy slang, probably from earlier sapskull (1735), saphead (1798), from sap as a shortened form of sapwood "soft wood between the inner bark and the heartwood" (late 14c.), from sap (n.1) + wood (n.); so called because it conducts the sap; compare sappy.
sap (v.1)youdaoicibaDictYouDict
"dig a trench toward the enemy's position," 1590s, from Middle French saper, from sappe "spade," from Late Latin sappa "spade" (source also of Italian zappa, Spanish zapa "spade"). Extended sense "weaken or destroy insidiously" is from 1755, probably influenced by the verb form of sap (n.1), on the notion of "draining the vital sap from." Related: Sapped; sapping.
sap (v.2)youdaoicibaDictYouDict
"hit with a sap," 1926, from sap (n.3). Related: Sapped; sapping.
sap (n.3)youdaoicibaDictYouDict
"club, stick for hitting," 1899, from shortening of sapwood (see sap (n.2)) or sapling.