expression, godan verb (-u)
speak badly of; bad-mouth
Used when criticizing someone or saying negative things about them, often behind their back.
彼はいつも人のことを悪く言う。
He always speaks badly of others.
陰で悪く言うのはよくない。
It's not good to bad-mouth people behind their backs.
悪口を言う is a more common and direct way to say 'speak ill of', while 悪く言う is slightly softer and less frequent.
Compound of 悪く (adverbial form of 悪い 'bad') + 言う 'say'. The exact historical derivation is straightforward.