If you say something acidly, you say it in a cruel or unkind way — sharp, as if the words could sting. The feeling is often polite on the surface and cutting underneath: a smile, then a remark that hurts. It is about tone of voice, or the tone of a written line, not about chemistry. You will meet it more in stories and news writing than in everyday chat; a friend might say she said it bitterly, or she was being snarky, instead. Here acid has its old figurative sense, “biting, harsh,” the way a sour taste bites. You would not use acidly to describe a chemical process.