"Abiding therein"—meaning: in the requital of that burden; and it is the Fire. "And evil it is for them"—meaning: and wretched it is for them—"on the Day of Resurrection as a burden"—referring to: what they carry upon their backs of the burden.
Muhammad said: "As a burden" (himlan) is in the accusative case (mansub) as a specifier (tamyiz). The meaning is: The burden is evil for them on the Day of Resurrection as a load. The burden (al-wizr) was named a load (himl) because its bearer is weighed down by it.