With little or no preparation or forethought:
— Trying to sound offhanded and reassuring.

synonyms: ad-lib, extemporaneous, extemporary, extempore, impromptu, off-the-cuff, offhand, unrehearsed.

Casually thoughtless or inconsiderate.

synonym: offhand.

Without previous thought or preparation:
— She had made these remarks offhandedly.

synonyms: offhand, offhandedly.

In a casually inconsiderate manner:
— She threw him over offhandedly without even a Dear-John letter.

synonyms: offhand, offhandedly.