Sentence:  …the enthusiastic borderism of Scott, who, rather than that a freebooter should under-act his part, or a bard <WSD>undersing</WSD> his praise, took the liberty of sometimes inventing the action and then resounding its glories, and debiting both to the account of originals, quite innocent of either. 
Meaning of the word marked with <WSD> tags:  To sing inadequately, or with too little vocal effort.