sap (n.1)

"juice or fluid which circulates in plants, the blood of plant life," Middle English sap, from Old English sæp, from Proto-Germanic *sapam (source also of Middle Low German, Middle Dutch, Dutch sap, Old High German saf, and, with unetymological -t, German Saft "juice"). This is reconstructed to be from PIE root *sab- "juice, fluid" (source also of Sanskrit sabar- "sap, milk, nectar," Irish sug, Russian soku "sap," Lithuanian sakas "tree-gum"). As a verb meaning "to drain the sap from," by 1725.

"simpleton," 1793, probably shortened from earlier words in related senses, such as sappy, sapskull (1735), saphead (1739). The notion perhaps is from sap (n.1) as suggestive of freshness or "greenness" (sapling in the extended sense of "young or inexperienced person" is attested from 1580s). But another theory links it to sap as a dialectal word for a mush or sop, and from there extended to mean a person of soft intellect or character.

Also as a verb, "to act like a sap."

1590s, intransitive, "dig a covered trench toward the enemy's position," from French saper, from sappe "spade," from Late Latin sappa "spade, mattock" (source also of Italian zappa, Spanish zapa "spade"), which is of unknown origin. The transitive sense of "undermine (a wall, etc.), render unstable by digging into or eating away the foundations" is from 1650s.

The extended transitive sense (of health, confidence, etc.), "weaken or destroy insidiously," is by 1755 and perhaps has been influenced or reinforced in sense by the verb form of sap (n.1), on the notion of "draining the vital sap from," and later by sap (v.2) "beat with a club or stick."

It also sometimes is used as a noun, "a narrow, covered ditch or trench by which a fortress or besieged place can be approached under fire" (1640s). Sap (n.) in the "tool for digging" sense also occasionally is met in 16th century English. Related: Sapped; sapping.

"club or stick for hitting," implied by 1899 in "Tramping With Tramps" (saps), and perhaps originally a word from that subculture; said in earliest references to be a shortening of sapling, which was noted by 1712 as something you could use as a weapon to beat someone with. Also sapstick (1915).

"hit with a sap," 1926, from sap (n.3). Hence, in a general sense, "to beat up." Related: Sapped; sapping.