The definition of spread oneself too thin is:
(verb) to be involved in too many different activities
Your son is not doing well in school, because he has spread himself too thin. He has too many hobbies!
Melyssa works two jobs, goes to school full-time and volunteers to help the homeless. She is really spreading herself too thin.
I was spreading myself too thin by playing basketball, football and baseball while trying to work on a Masters degree.
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