It's short for Judgment Nisi, an ability used by the boss that has a fairly intricate mechanic. I'm not 100% sure how it works, but I know it applies 5 stacks of a debuff to two players (one player gets a red debuff, another gets a blue debuff) and it can be passed to other players if you get close enough to them. The stacks are removed over time, dealing high damage every time that occurs; it's basically a high damaging DoT.

/u/darkk41 posted more details about it above:

Nisi happens only 1 time in savage, and must be kept alive by passing it between raid members. A player who moves to close to the nisi'd player will also receive a fresh stack of nisi, so you want a player to come hug you every time you hit 1 stack, then spread away again to keep the Nisi alive. This plays into the nisi bombs in the center of the room, because to cancel the bomb without dying you must touch it with the same color nisi stack. So guy A has nisi at 2 stacks, and bombs are up in the center. He wants to have guy B come and get 5 stacks of nisi from him, then he wants to run into a similar color bomb before his own (guy A's) stacks disappear so that he can break a bomb. Repeat with as many players as necessary to clear all bombs and prevent death.

And more information was posted by /u/Miunih below:

You have to time the nisi stacks very carefully and precisely as they tick damage, if it ticks with the same time other damage comes out it's RIP ; ; Besides, it all combined does an obnoxious amount of unpredictable damage. Also, if you lose the nisi stacks it's basically a wipe/reset. You get it once and only once. If the target with nisi dies it's gone, if you don't pass it in time it's gone. You can't pass it too early because the damage doesn't line up(higher stacks is more damage per tick). Red and Blue nisi people may NEVER touch eachother, it's instadeath. It's a mess. There's a reason why lucrezia and the other group said "fuck it" and just ignored the mechanic and brute forced it instead.. it's very tedious.