Offhand, a.
Instant; ready; extemporaneous; as, an offhand speech; offhand excuses. -- adv.
In an offhand manner; as, he replied offhand.
Shades of meaning
Fernald has no entry of its own for offhand; he lists it under extemporaneous.
Where he writes about it:
Offhand is still more emphatic as to the readiness and freedom of the utterance. — from extemporaneous