ad lib vs busk it

ad lib

adj

Extemporaneous, impromptu.

verb

To perform without preparation.
To perform without script.
To perform extemporaneously.

adv

Extemporaneously.
At will.
To whatever extent.
At pleasure.

busk it

How often have the words ad lib and busk it occurred in a corpus of books? (source: Google Ngram Viewer )