An adverbial mimetic meaning carelessly or inadvertently (e.g., to forget or overlook something because of inattention).
うっかりドアに鍵をかけ忘れてしまった。
I carelessly forgot to lock the door.
彼女はうっかり名前を呼び間違えた。
She inadvertently called someone by the wrong name.
会議に遅れないようにと言ったのに、うっかり寝坊した。
Even though I was told not to be late for the meeting, I accidentally overslept.
うっかりして財布を家に置いてきた。
I absentmindedly left my wallet at home.