"I will overburden him with ascent," meaning: I will burden him with the hardship of torment.
Muhammad said: An arduous mountain pass is called a 'su'ud' (ascent), and likewise a 'ka'ud' (steep ascent).
﴿سَأُرْهِقُهُ صعُودًا﴾ أَيْ: سَأَحْمِلُهُ عَلَى مَشَقَّةٍ مِنَ الْعَذَاب.
قَالَ محمدٌ: وَيُقَال الْعقبَة الشاقة: صعودٌ وَكَذَلِكَ الْكَئودُ.
