When you dye your hair pink and orange, your mother decries your act as a horror and bursts into tears. She criticizes your choice of colors, stating that pink and purple would have looked better.
You might decry learning French, declaring it as a waste of time. Yet many English words come from French. When the French conquered England in 1066, they brought with them their language as well as their social system. Because French was spoken mostly by the ruling class, many of the loan words are formal. Decry comes from the Old French descrier, "to cry out" or "announce." When you decry something, you simultaneously condemn it and discredit it, a formal act.