Sentence:  Such was the <WSD>impromptu</WSD> that Voltaire gave to howls of laughter at Sans Souci and that was soon circulated in manuscript throughout the literary circles of Europe , to be printed sometime later , but with the name of Timon of Athens , the famous misanthrope , substituted for that of Rousseau . 
Meaning of the word marked with <WSD> tags:  an extemporaneous speech or remark