careless person; absentminded person; thoughtless person
Describes someone who often makes careless mistakes, forgets things, or acts without thinking. The word is mildly critical but not strongly insulting.
He's a careless person and often forgets his keys.
うっかり者だから、大事な書類をなくしてしまった。
Being absentminded, I lost an important document.
Compound of the mimetic adverb うっかり (carelessly, absentmindedly) and 者 (person).