"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."
updated on December 21, 2021