The Quran

وَجاءَ المُعَذِّرونَ مِنَ الأَعرابِ لِيُؤذَنَ لَهُم وَقَعَدَ الَّذينَ كَذَبُوا اللَّهَ وَرَسولَهُ ۚ سَيُصيبُ الَّذينَ كَفَروا مِنهُم عَذابٌ أَليمٌ

Muhammad Asad

AND THERE came [unto the Apostle] such of the bedouin as had some excuse to offer, [with the request] that they be granted exemption,¹ whereas those who were bent on giving the lie to God and His Apostle [simply] remained at home.² [And] grievous suffering is bound to befall such of them as are bent on denying the truth!

I.e., from participating in the expedition to Tabuk. The term al-mu'adhdhiran connotes both "those having a valid excuse ('udhr)" and "those offering false excuses"; it is, therefore, best rendered as "such as had some excuse to offer". The specific mention of the a'rab ("bedouin") in this and the following passages probably arises from the fact that their attitude - positive or negative - towards Islam was of the greatest importance within the context of early Muslim history, inasmuch as the message of Muhammad could not obtain a real, lasting foothold in Arabia without first securing the allegiance of those warlike nomads and half-nomads, who constituted the great majority of the Peninsula's population. At the time when the Prophet was preparing to set out towards Tabuk, many of the already converted tribesmen were willing to go to war under his leadership (and, in fact, did so), while others were afraid lest in their absence their encampments, denuded of man-power, be raided by hostile, as yet unconverted tribes (Razi); others, again, were simply averse to exposing themselves to the hardships of a campaign in distant lands, which did not seem to them to have any bearing on their own, immediate interests.
I.e., without even caring to come to Medina and to excuse themselves.