offhand

Casually thoughtless or inconsiderate.

“an offhand manner”

“she treated most men with offhand contempt”

With little or no preparation or forethought.

“offhand excuses”

“trying to sound offhanded and reassuring”

Without previous thought or preparation.

“couldn't give the figures offhand”

“we decided offhand to go to Canada”

In a casually inconsiderate manner.

“replied offhand, his mind a million miles away”

“she threw him over offhandedly without even a Dear-John letter”