It's a humorous way to describe someone's mental toughness, as if their courage is strong enough to withstand even the most difficult situations, like the strength of one's intestinal tract.
It's often used in situations where someone needs to endure pain, hardship, or unpleasantness without giving in.
Here are some examples:
* "It took a lot of intestinal fortitude to stand up to the bully."
* "She showed incredible intestinal fortitude by finishing the race despite her injury."
* "The soldiers displayed incredible intestinal fortitude in the face of enemy fire."
It's important to note that the term "intestinal fortitude" is not used in a literal sense, but rather as a metaphor to describe someone's inner strength.