Synonyms for Deficient:

adj.

all (adjective)
lacking, wanting.
deficient (adjective)
inadequate, imperfect, lacking, insufficient.
diminished (adjective)
minus.
dissatisfactory (adjective)
unsuitable.
imperfect (adjective)
faulty, blemished, defective, inadequate, spoiled, flawed, tainted, imperfect, blotched, second-rate, incomplete.
imperfect, inadequate (adjective)
faulty, insufficient, unfinished, inferior, lacking, sketchy, scant, incomplete, scanty, defective, unsatisfactory, bad, skimpy, meager, wanting, short, flawed, scarce, shy.
incomplete (adjective)
lacking, unfulfilled, sketchy, insufficient, undone, fractional, short, segmental, partial, incomplete, discontinuous, missing, sectional, Curtailed, Unaccomplished, embryonic, fragmentary, unsatisfactory, unachieved, scanty, patchy, unattained, unfinished, immature.
insufficient (adjective)
insufficient, inadequate, miserly, sparse, poor, meager, Flyspeck, scanty, niggardly, skimpy, stingy, scarce.
minus (adjective)
minus.
short (adjective)
short.
small (adjective)
baby, small, dwarfish, sparing, trifling, paltry, wee, slim, smallish, nugatory, teeny, teeny-weeny, minute, stingy, miserly, modest, miniature, slight, scrawny, negligible, piddling, puny, picayune, scant, minor, spare, toy, compact, delicate, little, short, minimal, scanty, itty-bitty, teensy, petite, pocket, meager, niggardly, weeny, diminutive, bantam, microscopic, ungenerous, beggarly, tiny.
unacceptable (adjective)
unacceptable, insufficient, unsatisfactory, unsuitable, untenable, inadequate, intolerable, inferior, inadmissible, inappropriate.

n.

bad, embarrassing, pitiful, terrible, low-grade, disgraceful, dire. lack, want for, bereft of something, minus, starved, stretched, excess, devoid, better, deprived. big, under. deficient (noun)
lean, scarce, light, too little, shy, inferior, nonstandard, short, lacking, poor, inadequate, wanting, skimpy, scant, substandard, insufficient, low, depleted.

Other synonyms:

deprived, devoid, lack, stretched, want for. starved. under. Other relevant words:
lean, nonstandard, substandard, light, lack, wanting, depleted, sketchy, shy, starved, too little, under, low, devoid, bad, minus.

Usage examples for deficient

  1. And therefore I must insist that Virgil was no way deficient in Poetical Fire; and that Homer excelled him not in that Particular. – The Preface to the Aeneis of Virgil (1718) by Joseph Trapp
  2. He can possibly gain his; but you, having no brains to speak of- the fact that good- looking men are always deficient in that respect is a continual and blessed consolation to us plain ones, Staff- will have to make what the world calls a 'good marriage. – At Love's Cost by Charles Garvice