"It's Kwuarm Oil."
"Kwuarm? Didn't you--"
"Use it on you? Number of occasions, yes." Inara smiled coyly. "And, if I do remember, you liked it."
"Who couldn't?"
"You wouldn't believe what it's made from."
"What's it made from?"
"Animal on one of the rim planets, called a Kwuarm. It's the bile extracted from the Kwuarm's stomach."
-- One Small Favour - Part I by KayleeBird