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long, slender fish, c.1300, common Germanic, cf. Dutch leng, German Leng, Old Norse langa, probably ultimately related to long (adj.).
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 the suffixes represented by English -le (cf. icicle, thimble, handle), from Old English -ol, -ul, -el; and -ing, suffix indicating "person or thing of a specific kind or origin;" in masculine nouns also "son of" (cf. farthing, atheling, Old English horing "adulterer, fornicator").
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 (e.g. gæslingr "gosling"). Thus it is possible that the diminutive use that developed in Middle English is from Old Norse.