Definitions of behind:

• noun:   the fleshy part of the human body that you sit on
• adverb:   in or to or toward the rear

  Example: "He followed behind"

• adverb:   remaining in a place or condition that has been left or departed from

  Example: "When he died he left much unfinished work behind"

• adverb:   in debt

  Example: "He fell behind with his mortgage payments"

• adverb:   in or into an inferior position

  Example: "Fell behind in his studies"

• adverb:   of timepieces

  Example: "My watch is running behind"