The Joker

In the Joker movies, Joaquin Phoenix plays Arthur Fleck, a man with several mental illnesses, one of which causes him to laugh uncontrollably at inappropriate times. The condition isn't specifically named on screen, but it's likely based on a real disorder called pseudobulbar affect.

Pseudobulbar affect (PBA) is a neurological condition characterized by sudden, uncontrollable episodes of laughing or crying that are often inappropriate or disproportionate to the situation. 

Symptoms include the following:

• Sudden, involuntary outbursts of laughter or crying

• Episodes that can last from seconds to minutes

• Crying or laughing that may be inappropriate or exaggerated

• Embarrassment or distress caused by the episodes

• Difficulty controlling emotions 

She would laugh like that when she was overly nervous or stressed. It was terrifying.