From painstaking + -ly.
painstakingly (comparative more painstakingly, superlative most painstakingly)
- In a painstaking manner; very slowly and carefully.
1917, Mark Twain [pseudonym; Samuel Langhorne Clemens], “Is Shakespeare Dead?”, in What Is Man? And Other Essays, New York: Harper & Brothers, page 314:
I wish he had had a dog, just so we could see how painstakingly he would have divided that dog among the family, in his careful business way.
in a painstaking manner; very slowly and carefully