na-adjective
careless; thoughtless; inattentive
Describes a lack of attention or caution that leads to a mistake or oversight. Often used when someone misses something obvious or acts without thinking.
うかつにも彼の名前を忘れてしまった。
I carelessly forgot his name.
そんな大事なことを見落とすなんて、うかつだった。
It was careless of me to overlook such an important thing.
The word is of native Japanese origin. The kanji 迂 (roundabout, careless) and 闊 (broad, careless) are ateji applied later for their meanings.