If someone speaks or acts prissily, they are being very correct and very “proper”, and they are quick to look shocked at anything a bit rude, messy, or free — and the speaker does not like it. The word carries a small sting: we use it to show disapproval, not praise. It is the adverb from prissy, so it tells you how someone did a thing: she sat prissily; he said it prissily. You hear it more in stories and in slightly teasing talk than in careful formal writing. A polite person can still laugh and sit on the grass; someone who does it prissily makes a little show of being above all that.