Synonyms for Insufficient:

adj.

all (adjective)
deficient.
defective (adjective)
lacking.
deficient (adjective)
inferior, skimpy, deficient, imperfect, lacking, inadequate, faulty.
dissatisfactory (adjective)
unacceptable, unsuitable.
failing (adjective)
scanty.
feeble (adjective)
low.
few (adjective)
niggardly, lean, scarce, minimum, miserly, sparse, spotty, stingy, few, negligible, minimal, little, scanty.
imperfect (adjective)
faulty.
incomplete (adjective)
fragmentary, partial, Unaccomplished, scanty, incomplete, immature, fractional, unachieved, lacking, embryonic, unsatisfactory, short, unfinished, unfulfilled, undone, patchy, sectional, missing, deficient, unattained, segmental, discontinuous, Curtailed.
insufficient (adjective)
unsatisfactory, scant, scarce, meager, imperfect, Flyspeck, incomplete, faulty, deficient, short, stingy, poor, niggardly, unfinished, shy, scanty, inadequate, miserly, sparse, skimpy.
poor (adjective)
niggardly.
unacceptable (adjective)
inadequate, inappropriate, deficient, intolerable, inferior, inadmissible, untenable, unsatisfactory, unacceptable, unsuitable.

n.

miserable, pitiful, tight, derisory, in short/limited supply, meagre, not go far. faulty. insufficient (noun)
shy, depleted, too little, light, short, poor, scant, scarce, deficient, low, lean, inadequate, skimpy.

Other synonyms:

derisory. under. miserable. light. tight.

Usage examples for insufficient

  1. Late nights and insufficient sleep, added to the need of a shave, always made him look like something that should have been swept up and taken away to the ash- bin. – A Damsel in Distress by Pelham Grenville Wodehouse
  2. And, Mr. Fitzgerald, you must allow me to say, that your income is altogether insufficient for her wants and your own. – Castle Richmond by Anthony Trollope
  3. For the formation of very thick beds of salt, and especially of thick beds of fairly pure composition, however, this simple explanation of conditions is insufficient – The Economic Aspect of Geology by C. K. Leith