The Undercommon which is also known as Deep Speech and it is the trade language of the underdark. Almost all of the intelligent underdark creatures speak the undercommon. This 5e language is the combination of drow language, dwarven and also the surface common even it has the mixture of some goblin language and the other lesser words thrown in.
But it is basically based mainly off the drow language originally. This language was originally created by the slaves of the drow which is to communicate between each other like slaves of various races began interacting. It eventually grew to the point which is became a trade language within the “lesser” underdark races, since so many could speak at least one of it.
Basically, this undercommon language is reluctant to speak to the drow, since this existence of this tongue is basically an insult to the Xanalress. An alternative to the drow of Arelith is putting in the “drow words” in their sentences, also even when they speak common, to give it an extra touch.
With this undercommon language the gnolls do not know like what for they are not originally from this underdark, nor have drow kept them as the slaves often.