Source : Webster's Revised Unabridged Dictionary (1913)

Accidentally \Ac`ci*den"tal*ly\, adv.
   In an accidental manner; unexpectedly; by chance;
   unintentionally; casually; fortuitously; not essentially.

Source : WordNet®

accidentally
     adv 1: without advance planning; "they met accidentally" [syn: {by
            chance}, {circumstantially}, {unexpectedly}] [ant: {intentionally}]
     2: in an incidental manner; "these magnificent achievements
        were only incidentally influenced by Oriental models"
        [syn: {incidentally}, {by chance}]
     3: without intention; in an unintentional manner; "she hit him
        unintentionally" [syn: {unintentionally}] [ant: {intentionally},
         {intentionally}]