I would like to go on the record as saying: there is not such thing as a monokini. And even if there was, it would not be this:
Secondly, the 'bi' in bikini does not mean 'two" (as in two pieces). It was named after the site of an A Bomb test at the Bikini Islands. Which are named, best as anyone can tell, after the shape of texture of a coconut ("pikinni").
And thirdly, some one already coined the neologism "monokini" back in 1960 to refer to wearing only the bottom part of a bikinis, a.k.a. topless bikini.
Thus endeth the rant.