noun

• (graphical user interface) The part of a slider that may be moved linearly along the slider.
  "a scroll-bar thumb"
• (colloquial, Internet) A thumbnail picture.

verb

• (transitive) To touch or cover with the thumb.
  "to thumb the touch-hole of a cannon"
• (transitive, with through) To turn the pages of (a book) in order to read it cursorily.
  "thumb through"
• (travel) To hitchhike.
  "So I started thumbin' back east, toward my hometown."

name

• (informal) The eastern part of the Lower Peninsula of Michigan, United States (the "Mitten").
  "Turnip Rock is located in a portion of eastern Michigan dubbed the Thumb, where 140 miles of coastline and waterfront towns offer a variety of outdoor activities, including biking, walking and kayaking."