Antonyms of energetic:
lethargic, unergetic, languid, logy, groggy, unenterprising, unenrgetic, lackadaisical, foggy, nonenterprising, dreamy, listless, dazed, languorous, stuporous, dull, inactive, lazy, slow, heavy, indolent, quiescent, sluggish, idle, inert, quiet, stupid, delicate, effete, enervated, faint, feeble, frail, infirm, wan, weak, weakened, impotent, powerless, prostrate, prostrated, sapped, tired, invertebrate, nerveless, soft, spineless, wimpy, ill, unhealthy, unsound, unwell, broken-down, debilitated, decrepit, disabled, wasted, worn-out, torpid, unambitious, sleepy, weary, apathetic, impassive, phlegmatic, stolid, boring, irksome, tedious, dead, inanimate, languishing, leaden, lifeless, limp, spiritless, vapid, guarded, mild, uncompelling, wishy-washy, ambiguous, equivocal, halting, hesitant, understated, nonassertive, nonemphatic, unemphatic.
Examples of usage:
  1. Under his less energetic successor, it was taken in a day.
    "Account of a Tour in Normandy, Vol. II. (of 2)", Dawson Turner.
  2. I established with myself, on these occasions, the reputation of a first- rate man of business,- prompt, decisive, energetic, clear, cool- headed.
    "Great Expectations", Charles Dickens.
  3. " I really must be a little more energetic, or I'll never find anything to suit me," he smiled in his most leisurely manner.
    "Simon", J. Storer Clouston.