Tafsir of As-Saff 61:3

This is an exposition of the magnitude of the repulsiveness of their actions. The verb "كبر" (is great/grave) is of the same category as "بئس" (is evil); it contains an ambiguous pronoun clarified by the indefinite noun that follows it. "أن تقولوا" (that you say) is the specific object of the censure. It is also permitted that the pronoun in "كبر" refers to the verbal noun understood from His saying (the Exalted), "Why do you say?"—meaning: this saying is great in hatred. In this case, "أن تقولوا" serves as a substitute for the implied pronoun or as the predicate of an omitted subject.

It has been said that it is intended to express wonder (ta'ajjub), even without the specific syntactic structure for it, as in the verse: "And [regarding] the neighbor of Jassas... the tusk of Kulayb has struck." The meaning of this wonder is to magnify the matter in the hearts of the listeners. The phrase "أن تقولوا" is attributed, and "مقتا" (hatred) is placed in the accusative case as a clarification, indicating that their saying "what they do not do" is pure hatred, containing no mixture [of anything else], due to the extreme degree to which hatred is embedded in it.

The term maqt (hatred) was chosen because it is the most intense form of hatred and the most eloquent expression of it; from this comes nikah al-maqt (the marriage of hatred), referring to a man marrying his father's wife. It was not deemed sufficient to simply make the hatred "great," but rather it was made the most intense and foul of hatreds. The phrase "in the sight of Allah" is even more emphatic than that, for if its great hatred is established in the sight of Allah (the Exalted)—before whom all that is great becomes insignificant—then its magnitude and intensity are complete, and all doubts regarding it are dispelled.

Interpreting maqt in the way you have heard is the view of more than one scholar of language. Ibn Atiyyah said: Maqt is hatred stemming from a sin, suspicion, or baseness committed by the one who is hated. Al-Mubarrad said: A man is mamqut (hated) and maqit (odious) when everyone hates him. The verse is used as evidence for the obligation of fulfilling a vow. It is narrated from some of the predecessors that it was said to him, "Narrate to us," but he remained silent. When told again, "Narrate to us," he replied, "Do you command me to say what I do not do, and thus hasten the hatred of Allah (the Almighty and Majestic)?"