sprezzatura
The art of performing a difficult task so gracefully that it looks effortless; calculated nonchalance.
uncountable
[…] I have found quite a universal rule which in this matter seems to me valid above all others, and in all human affairs whether in word or deed: and that is to avoid affectation in every way possible as though it were some very rough and dangerous reef; and (to pronounce a new word perhaps) to practice in all things a certain sprezzatura [nonchalance]^([sic]), so as to conceal all art and make whatever is done or said appear to be without effort and almost without any thought about it.
Where it comes from
Borrowed from Italian sprezzatura (“nonchalance”), which was coined by Baldassare Castiglione in 1528. The term gained currency in English in the mid-20th century, often to describe art.
Adapted from the Wiktionary entry sprezzatura (CC BY-SA 4.0).