Stinksap is a green fluid that smells liked rancid manure and is contained in some plants like Mimbulus Mimbletonia, as well as certain trees. The Mimbulus Mimbletonia plant uses it as a defensive mechanism when touched, while Stinksap itself can be used to nurse ill animals.
On 1 September, 1995, Neville Longbottom showed his recently-acquired Mimbulus Mimbletonia to his classmates on the Hogwarts Express. In order to demonstrate its defensive mechanism, Neville prodded the plant, which covered the whole compartment in Stinksap. Harry was covered in it, Ginny blocked herself in time, and Luna's issue of The Quibbler was dirtied. Ginny used a spell to clean up the mess. 
In 1996, Aragog, leader of the Forbidden Forest Acromantula colony, fell ill. In an attempt to nurse the Acromantula back to health, Rubeus Hagrid extracted Stinksap from a cluster of trees found en route to Hogsmeade. Hagrid later administred the Stinksap, but was ultimately unable to heal the ailing Aragog. 
- Harry Potter and the Order of the Phoenix (First appearance)
- Harry Potter and the Order of the Phoenix (video game) (Mentioned only)
- Harry Potter and the Half-Blood Prince (film) (Mentioned only)