mid-15c., "infant at the breast," from suck + diminutive suffix -ling. Similar formation in Middle Dutch sogeling, Dutch zuigeling, German Säugling. Meaning "calf or other young mammal" is from 1520s. Meaning "act of breast-feeding" is attested from 1799. Adjectival sense "not yet weaned" is from 1799.
Entries linking to suckling
Old English sucan "to suck," from a Proto-Germanic word of imitative origin (cognates: Old Saxon, Old High German sugan, Old Norse suga, Danish suge, Swedish suga, Middle Dutch sughen, Dutch zuigen, German saugen "to suck"), possibly from the same source as Latin sugere "to suck," succus "juice, sap;" Old Irish sugim, Welsh sugno "to suck;" see sup (v.2). As a noun from c. 1300.
Meaning "do fellatio" is first recorded 1928. Slang sense of "be contemptible" first attested 1971 (the underlying notion is of fellatio). Related: Sucked; sucking. Suck eggs is from 1906. Suck hind tit "be inferior" is American English slang first recorded 1940.
The old, old saying that the runt pig always sucks the hind teat is not so far wrong, as it quite approximates the condition that exists. [The Chester White Journal, April 1921]
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.