From accidental +‎ -ly.

• IPA^(key): /ˌæk.sɪˈdɛnt(ə)li/, enPR: ăk-sĭ-dĕntʹ(ə)lē

• Audio (US):

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accidentally (comparative more accidentally, superlative most accidentally)

1. In an accidental manner; by chance, unexpectedly.

   Synonyms: casually, fortuitously; see also Thesaurus:accidentally

   He discovered penicillin largely accidentally.

2. Unintentionally.

   He accidentally exposed the bacteria to mold spores.

   • 1899, Knut Hamsun, “Part I”, in George Egerton [pseudonym; Mary Chavelita Dunne Bright], transl., Hunger […], London: Leonard Smithers and Co […], →OCLC, page 15:

     As I arrived at the foot of the hill I overtook two ladies, whom I passed. As I did so, I brushed one of them accidently on the arm.

   • 2011 December 14, Steven Morris, “Devon woman jailed for 168 days for killing kitten in microwave”, in Alan Rusbridger, editor, The Guardian‎^[1], London: Guardian News & Media, →ISSN, →OCLC:

     Robins, of Torquay, had denied a single charge of causing unnecessary suffering to an animal. She claimed the microwave was accidentally turned on by one of the cats after the kitten got inside. But Knutton said the kitten was too small to even get onto the work surface.

     (Can we archive this URL?)

• accidently (now nonstandard)

• intentionally, deliberately, designedly, on purpose, purposely, advisedly, by choice, by design

• accident