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sap [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Its original literal sense ‘dig a trench or tunnel underneath in order to attack’ has now been largely superseded by the metaphorical ‘weaken’, which has been heavily influenced by sap ‘plant-juice’ (from the notion of ‘draining sap from a plant’).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.2): "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 (n.3): "club, stick for hitting," 1899, from shortening of sapwood (see sap (n.2)) or sapling.