1781, "to make small, reduce in proportion," from be- + little (v.); first recorded in writings of Thomas Jefferson (and probably coined by him), Jefferson used it in "Notes on the State of Virginia" to characterize the view promoted as scientific by French naturalist Georges-Louis Leclerc Buffon that American species (including humans) were naturally smaller than and inferior to European ones, which Jefferson was at pains to refute. ("So far the Count de Buffon has carried this new theory of the tendency of nature to belittle her productions on this side of the Atlantic.") The word was roundly execrated in England, as be- is properly to be used only with verbs:
Belittle! What an expression! It may be an elegant one in Virginia, and even perfectly intelligible; but for our part, all we can do is to guess at its meaning. For shame, Mr. Jefferson! [European Magazine and London Review, August 1787; to guess was considered another Yankee barbarism]
Jefferson also sent Buffon a stuffed moose. The figurative sense of "depreciate, scorn as worthless" (as the reviewers did to this word) is from 1797 and is now almost the only sense. Related: Belittled; belittling.
Jefferson, as if disposed to assail the sovereignty of the English tongue as well as the sovereignty of the English sword, never hesitated to coin a word when it suited his purposes so to do; and though many of his brood are questionable on the ground of analogy and as intermixing languages; yet they were expressive, and became familiar. [Hugh Blair Grigsby, "The Virginia Convention of 1776," 1855]
Old English lytlian "to lessen, decrease, become little or less, diminish; shorten; fall out of use; belittle," from root of little (adj.).
late 14c., "degrade socially" (for marrying below rank or without proper ceremony), from Anglo-French and Old French desparagier (Modern French déparager) "reduce in rank, degrade, devalue, depreciate," originally "to marry unequally, marry to one of inferior condition or rank," and thus, by extension, to bring on oneself or one's family the disgrace or dishonor involved in this, from des- "away" (see dis-) + parage "rank, lineage" (see peer (n.)).
Also from late 14c. as "injure or dishonor by a comparison," especially by treating as equal or inferior to what is of less dignity, importance, or value. Sense of "belittle, undervalue, criticize or censure unjustly" is by 1530s. Related: Disparaged; disparaging; disparagingly.