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.