I have seen (in an online conversation) the idiom think nothing of sth for the first time. (The context was more-or-less something like this: "I am surprised that intelligent people who think nothing of calculating complicated integrals, cannot master basics of English grammar." This is just a paraphrase, you can replace integrals with any ability which is considered complicated and English grammar with anything considered relatively easy.)
I have seen the phrase think nothing of it in Wiktionary, when it is used as form of saying thank you. But this seems to be a different idiom.
When I searched for "who think nothing of", most occurrences I see could be understood as "who do not mind doing something". Although the first pages of search results are with negative connotations.