Overextend oneself, undertake too many different enterprises. For example, Tom's exhausted; what with work, volunteer activities, and social life he's spread himself too thin. This expression alludes to smearing something (like butter on bread) in such a thin layer that it does not cover the surface. Jonathan Swift used spread thin in a positive sense, that is, something should occur less often (Polite Conversation, 1731–1738): “They [polite speeches] ought to be husbanded better, and spread much thinner.”

To engage in so many activities that one can't perform any of them well: “Last semester, Pamela tried to play basketball, serve on the student council, and work at her father's store. She quickly discovered that she was spreading herself too thin.”