Zenzizenzizenzic is the eighth power of a number. It’s long obsolete, so much so that the Oxford English Dictionary only has one citation for it, from a famous work by the Welsh-born mathematician Robert Recorde, The Whetstone of Wit, published in 1557. It turns up from time to time as one of those weird words which is best known for being held up as an example of a weird word.
The root word, also obsolete, is zenzic. This was borrowed from German (the Germans were very big in algebra in the fourteenth and fifteenth centuries). They got it from the medieval Italian word censo, which is a close relative of the Latin census. The Italians (who were big in algebra even earlier) used censo to translate the Arabic word mál, literally “possessions; property”, which was the usual word in that language for the square of a number. This came about because the Arabs, like most mathematicians of those and earlier times, thought of a squared number as a depiction of an area, especially of land, hence property. So censo, and later our English zenzic, was for a while the word for a squared number.
Even by Robert Recorde’s time, there was no easy way of denoting the powers of numbers, a great hindrance to effective mathematics. The only term he had apart from the square was the cube, the third power of a number, and formulae were usually written out in words. Recorde, like his predecessors, represented a fourth power by the square of a square, zenzizenzic, which is just a condensed form of the Italian censo di censo, used by Leonardo of Pisa in his famous book Liber Abaci of 1202. An eighth power was by obvious extension zenzizenzizenzic. And similarly the sixth power was zenzicube, the square of a cube. None of these words survives in the language except as historical curiosities.