accidentally

from WordNet (r) 3.0 (2006)

accidentally
    adv 1: without advance planning; "they met accidentally" [syn:
           {by chance}, {accidentally}, {circumstantially},
           {unexpectedly}] [ant: {advisedly}, {by choice}, {by
           design}, {deliberately}, {designedly}, {intentionally},
           {on purpose}, {purposely}]
    2: of a minor or subordinate nature; "these magnificent
       achievements were only incidentally influenced by Oriental
       models" [syn: {incidentally}, {accidentally}]
    3: without intention; in an unintentional manner; "she hit him
       unintentionally" [syn: {unintentionally}, {accidentally}]
       [ant: {advisedly}, {by choice}, {by design}, {deliberately},
       {designedly}, {intentionally}, {on purpose}, {purposely}]

from The Collaborative International Dictionary of English v.0.48

Accidentally \Ac`ci*den"tal*ly\, adv.
   In an accidental manner; unexpectedly; by chance;
   unintentionally; casually; fortuitously; not essentially.
   [1913 Webster]