What does « Nuʿās » mean in the Quran?

Then after distress, He sent down upon you security [in the form of] drowsiness, overcoming a faction of you, while another faction worried about themselves, thinking of Allah other than the truth - the thought of ignorance, saying, "Is there anything for us [to have done] in this matter?" Say, "Indeed, the matter belongs completely to Allah." They conceal within themselves what they will not reveal to you. They say, "If there was anything we could have done in the matter, some of us would not have been killed right here." Say, "Even if you had been inside your houses, those decreed to be killed would have come out to their death beds." [It was] so that Allah might test what is in your breasts and purify what is in your hearts. And Allah is Knowing of that within the breasts.

[Remember] when He overwhelmed you with drowsiness [giving] security from Him and sent down upon you from the sky, rain by which to purify you and remove from you the evil [suggestions] of Satan and to make steadfast your hearts and plant firmly thereby your feet.