Source : Webster's Revised Unabridged Dictionary (1913)

Red-hand \Red"-hand`\ (r?d"h?nd`), Red-handed \Red"-hand`ed\
   (-h?nd`?d), a. or adv.
   Having hands red with blood; in the very act, as if with red
   or bloody hands; -- said of a person taken in the act of
   homicide; hence, fresh from the commission of crime; as, he
   was taken red-hand or red-handed.

Source : WordNet®

red-handed
     adj : in the act of committing a crime or other reprehensible act;
           "caught red-handed"
     adv : doing something reprehensible or showing clear evidence of
           having done something reprehensible; "he was caught
           red-handed"