sappy (adj.)

"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," 1815 (Scott), a word appearing at first especially in Scottish English and in English schoolboy slang, probably shortened from earlier words in related senses, such as sappy, sapskull (1735), saphead (1798). 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). Or perhaps it is a shortening of sapwood "soft wood between the inner bark and the heartwood" (so called because it conducts the sap), which was considered inferior material in carpentry.

The Joyner, though an honest man, yet hee maketh his joynts weake, and putteth in sap in the mortesels, which should be the hart of the tree, and all to make his stuffe slender. [Robert Greene, "A Quip for an Upstart Courtier," 1592]

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

"young tree," early 14c., from sap (n.1) + diminutive suffix -ling. Especially a young forest tree when the trunk is about three or four inches across. This probably is the source of American English slang sap (n.3) "club, short staff" and the verb sap (v.2) "to hit (someone) with a sap." By 1580s as "young or inexperienced person."