Words related to sapling
"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.
Both these suffixes had occasional diminutive force, but this was only slightly evident in Old English -ling and its equivalents in Germanic languages except Norse, where it commonly was used as a diminutive suffix, especially in words designating the young of animals (such as gæslingr "gosling"). Thus it is possible that the diminutive use that developed in Middle English is from Old Norse.
"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).
"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."
Middle English sapi, of a tree or of wood, "full of sap," from Late Old English sæpig, from sæp "sap of a plant" (see sap (n.1)). The colloquial figurative sense, in reference to persons, etc., "foolish, foolishly sentimental" (1660s) might have developed from an intermediate sense of "too wet, sodden, soggy" (late 15c.), or it might have come from sappy as "containing sapwood" (mid-15c.); compare sap (n.2). Or it might be from the notion of "green, juvenile," like a sapling tree. Earlier, now obsolete, figurative senses were "full of vitality" (1550s) and "immature" (1620s). Related: Sappily; sappiness.