"R — r — riddikulus! "squeaked Neville.
There was a noise like a whip crack. Snape stumbled; he was wearing a long, lace-trimmed dress and a towering hat topped with a moth-eaten vulture, and he was swinging a huge crimson handbag.
There was a roar of laughter; the boggart paused, confused, and Professor Lupin shouted, “Parvati! Forward!”
The first thing Snape asks Harry is “Potter! What would I get if I added powdered root of asphodel to an infusion of wormwood?” According to Victorian Flower Language, asphodel is a type of lily meaning ‘My regrets follow you to the grave’ and wormwood means ‘absence’ and also typically symbolized bitter sorrow. If you combined that, it meant ‘I bitterly regret Lily’s death’.