[syn: unintentionally, accidentally]
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]
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]