late 14c., "serving to effect the intended purpose," from Old French effectif, from Latin effectivus "productive, effective," from effect-, stem of efficere "work out, accomplish" (see effect (n.)). Of military forces, "fit for action or duty," from 1680s.
-ness
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).