Synonyms for Disappointing:

adj.

all (adjective)
dissatisfactory, unsatisfying.
dissatisfactory (adjective)
unsatisfying, insufficient, lame.
unsatisfactory (adjective)
unsatisfying, distasteful, falling short, displeasing, disconcerting, inadequate, ineffective, insufficient, unpleasant, frustrating, failing, discouraging, disagreeable, depressing, disheartening, inferior, lame, bitter, unhappy, uninteresting, mediocre, unfulfilling.

n.

suboptimal, average, substandard, poor, undistinguished. unlucky, happy, sorry. disappointing (noun)
unsatisfying, dissatisfactory, unsatisfactory.
unsatisfactory (noun)
unfulfilling, disagreeable, frustrating, unexpected, ineffective, depressing, inferior, deplorable, unlooked-for, unpleasant, insufficient, second-rate, lame, vexing, unlucky, failing, distasteful, inadequate, short of expectations, unhappy, falling short, uninteresting, mediocre, bitter, limited.

v.

disappointing (verb)
dashing, disheartening, disconcerting, disillusioning, discouraging, thwarting.
displeasing (verb)
displeasing, irritating, disgusting, jarring, Aggrieving, Scandalizing, grating, Bothering, Angering, offending, disturbing, repelling, annoying, Riling, shocking, provoking.

Other synonyms:

suboptimal, mediocre, undistinguished, substandard. inferior. unlucky. poor, sorry. Other relevant words:
vexing, frustrating, dissatisfactory, mediocre, limited, bitter, average, unsatisfying, lame, unsatisfactory, second-rate, unhappy, depressing, unfulfilling, uninteresting, falling short, distasteful, sorry, substandard, undistinguished, insufficient, deplorable, unlucky, poor, disagreeable, inadequate, unpleasant, failing, inferior, unexpected, happy, suboptimal, ineffective.

Usage examples for disappointing

  1. 30 When you have done a man a favor do not insist too earnestly that it is a mere trifle, or he may take you at your word and not trouble to repay it; which would be very disappointing – Crankisms by Lisle de Vaux Matthewman
  2. Ferrier made a very disappointing speech. – The Testing of Diana Mallory by Mrs. Humphry Ward