offhand
off-hand
adverb
Without previous thought or preparation; spontaneously or from immediate memory.
"I can't tell you offhand."
adjective
Said or done without much thought or preparation; casual or unconcerned.
"His offhand remark upset her."
Thesaurus
Synonyms: offhanded, careless, offhandedly, impromptu, extempore, unprepared, unrehearsed, extemporaneous, ad-lib, off-the-cuff, extemporary