うっかり

adverb, adverb taking the 'to' particle, noun or participle which takes 'suru'

carelessly; inadvertently; absent-mindedly

Describes an unintentional mistake or oversight due to lack of attention; often used with verbs like 忘れる, 言う, or する.

• うっかり約やく束そくを忘わすれてしまった。

  I carelessly forgot about the appointment.