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Lit., "thou shalt see [or "behold"] mankind drunk", i.e., behaving as if they were drunk. The illusory, purely subjective character of this "seeing" - implied in the use of the singular form tara ("thou shalt see") after the plural "you" employed in the first clause of this verse - justifies the rendering "it will seem to thee that...", etc.
My interpolation of the words "their dread of" is based on the statement in 21:103 that, as far as the righteous are concerned, "the supreme awesomeness [of the Day of Resurrection] will cause them no grief" despite the dread with which it will overwhelm every human being.
Three metaphors are used for the extreme terror which the Awful Day will inspire, (1) No mother abandons the baby at her breast in the greatest danger; yet that will happen in this Dreadful Hour. (2) An expectant mother carries the young life within her with great pride and hope: yet the terror will overpower the hope at this "Hour", and nature's working will be reversed, (3) Men ordinarily retain their self-possession except under intoxication: here, without intoxication, they will be driven to frenzy with terror.