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."