(Fee-LICKS Fi-li-KUS) is the name of a fictional potion in the book Harry Potter and the Half-Blood Prince that grants temporary good luck to the drinker. The name, like many things in the Harry Potter novels, is Latinate in origin, and translates to "lucky of lucky," or "happy of happy." ("happy", strictly speaking, means "lucky.")
Harry: "With this Felix Felicis, I can do anything..."
Snape: "How about shutting the hell up!?"