early 14c., from sap (n.1) + diminutive suffix -ling. This probably is the source of American English slang sap (n.3) "club, short staff" (1899) and the verb sap (v.2) "to hit (someone) with a sap" (1926).
"liquid in a plant," Old English sæp, from Proto-Germanic *sapam (source also of Middle Low German, Middle Dutch, Dutch sap, Old High German saf, German Saft "juice"), 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," 1725.
diminutive word-forming element, early 14c., from Old English -ling a nominal suffix (not originally diminutive), from Proto-Germanic *-linga-; attested in historical Germanic languages as a simple suffix, but probably representing a fusion of two suffixes: 1. that represented by English -el (1), as in thimble, handle; and 2. -ing, suffix indicating "person or thing of a specific kind or origin;" in masculine nouns also "son of" (as in farthing, atheling, Old English horing "adulterer, fornicator"), from PIE *-(i)ko- (see -ic).
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.