Hippopotomonstrosesquippedaliophobia is the fear of what?

Long words.

Link for above:

Hippopotomonstrosesquippedaliophobia is one of the longest words in the dictionary, and ironically, it means the fear of long words. It originally was referred to as Sesquipedalophobia but was changed at some point to sound more intimidating.

https://psychcentral.com/anxiety/hippopotomonstrosesquippedaliophobia

Hippopotomonstrosesquippedaliophobia is not officially recognized as a diagnosis. Neither the American Psychiatric Association nor the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) outline a specific definition for this phobia but rather regard it as a subtype of social phobia: People may have feelings of shame or fear of ridicule for mispronouncing long words for example in social situations where a person may be examined, like meeting new people or having a conversation. This feeling may cause distress or anxiety.

https://www.healthline.com/health/hippopotomonstrosesquippedaliophobia