mid-14c., "short or insufficient in quantity, rather less than is wanted for the purpose," from a Scandinavian source such as Old Norse skamt, neuter of skammr "short, brief," from Proto-Germanic *skamma- (source also of Old English scamm "short," Old High German skemmen "to shorten"), perhaps ultimately "hornless" (from PIE *kem- (1) "hornless;" see hind (n.)).
Also in Middle English as a noun, "dearth, scant supply, scarcity," from Old Norse.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).
<a href="https://www.etymonline.com/word/scantness">Etymology of scantness by etymonline</a>
Harper, D. (n.d.). Etymology of scantness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/scantness