compactness (n.)

late 14c., of substances, "closely and firmly united," from Latin compactus "concentrated," past participle of compingere "to fasten together, construct," from com "with, together" (see com-) + pangere "to fix, fasten" (from PIE root *pag- "to fasten"). Related: Compactly; compactness. Compact car is 1960. Compact disc is from 1979.

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).