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The verb ahfa means "he did [a thing] in an excessive measure" or "he exceeded the usual bounds in doing [something]". In connection with an inquiry, and especially when followed by 'anhu or 'anha ("about it"), it signifies "he tried hard to gain insight [into something] by persistently inquiring about it". Thus, used as a participle, it means "one who has gained insight [into something] through persistent inquiry". In the above context, the implication is that no amount of inquiry or speculation can reveal to man - the prophets included - the coming of the Last Hour before its actual manifestation.
Hafi is usually construed to mean; "eager or anxious in search of": the preposition following here is 'an = concerning, about. Some commentators (including Ragib) understand it in this passage to mean "well-acquainted." In xix. 47, with the preposition by following it, it signifies "well-disposed to", favourable to, good to, kind to."
The fact of its coming is a certainty: the exact time appointed for it is not revealed by Allah. If it were it would be so momentous as to disturb our thoughts and life. It would be a heavy burden to us. Our duty is to be prepared for it at all times. It will come when we least expect it. In the present Gospels Jesus says the same thing: he does not know the Hour, but it will come suddenly. "But of that day and that Hour knoweth no man, no, not the angels which are in heaven, neither the Son, but the Father. Take ye heed, watch and pray: for ye know not when the time is." (Mark. xiii. 32-33).