Source : Webster's Revised Unabridged Dictionary (1913)

Sinful \Sin"ful\, a. [AAS. synfull.]
   Tainted with, or full of, sin; wicked; iniquitous; criminal;
   unholy; as, sinful men; sinful thoughts. --Piers Plowman.

         Ah sinful nation, a people laden with iniquity. --Isa.
                                                  i. 4.
   -- {Sin"ful*ly}, adv. -- {Sin"ful*ness}, n.

Source : WordNet®

sinful
     adj 1: characterized by iniquity; wicked because it is believed to
            be a sin; "iniquitous deeds"; "he said it was sinful
            to wear lipstick"; "ungodly acts" [syn: {iniquitous},
            {ungodly}]
     2: having committed unrighteous acts; "a sinful person" [syn: {unholy},
         {wicked}]