Tafsir of Al-Kahf 18:73

{بما نسيت} "By what I have forgotten": Meaning that which I forgot, or something I forgot, or by my act of forgetting. He intended that he had forgotten his instruction, and there is no accountability for one who forgets.

Alternatively, he phrased the speech in the form of a prohibition against holding him accountable for forgetfulness, while implying that he had indeed forgotten, in order to lay out his excuse for the objection. This is among the ma‘arid (allusive/indirect) forms of speech used to avoid lying while still achieving one's purpose, similar to the saying of Abraham: "This is my sister," and "Indeed, I am ill."

Or, he meant by "forgetfulness": abandonment. That is: "Do not hold me accountable for what I abandoned of your instruction the first time."

{ولا ترهقني من أمري عسرا} It is said: rahiqa-hu (he overtook him) if he covered him, and arhaqa-hu iyyahu (he caused him to be covered by it). Meaning: "Do not cover me with difficulty in my affair." This refers to his following him; he means: "Do not make following you difficult for me; rather, make it easy for me by overlooking [my mistakes] and refraining from scrutiny."

It is also recited as ‘usuran (with two dammahs).

{فانطلقا حتى إذا لقيا غلاما فقتله قال أقتلت نفسا زكية بغير نفس لقد جئت شيئا نكرا * قال ألم أقل لك إنك لن تستطيع معى صبرا}