Definitions of fellow:

• noun:   an informal form of address for a man

  Example: "Say, fellow, what are you doing?"

• noun:   a man who is the lover of a girl or young woman
• noun:   a boy or man

  Example: "There's a fellow at the door"

• noun:   a person who is member of your class or profession

  Example: "He sent e-mail to his fellow hackers"

• noun:   a person who is frequently in the company of another