To knowingly say that something is smaller or less important than it actually is.
English Synonyms and Antonyms, by James Champlin Fernald
To decry is to cry down, in some noisy, public, or conspicuous manner. A witness or a statement is discredited; the currency is depreciated; a good name is dishonored by unworthy conduct; we underestimate in our own minds; we may underrate or undervalue in statement to others. These words are used, with few exceptions, of things such as qualities, merits, attainments, etc. To disparage is to belittle by damaging comparison or suggestion; it is used only of things. A man's achievements are disparaged, his motives depreciated, his professions discredited; he himself is calumniated, slandered, etc. Compare SLANDER.