He struck him with the skulls of his head. رماه بأقحاف رأسه.

That is, he silenced him with a calamity he inflicted upon him. The plural form is used because they meant he struck him repeatedly. It is also possible to use the plural with what surrounds him, implying that every part of him is a skull, just as they say: thick-lipped, broad-shouldered. The skull is the part of the head that covers the brain. One cannot strike him with it without dislodging it from its place and removing it. This is a metaphor for killing him, as if he reached the ultimate limit of silencing him, which is death. And the slain cannot speak.

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The closest English equivalent to this proverb is:
He silenced him completely.
Meaning: He silenced him completely, leaving him with no answer.
An even stronger expression, and one that closely resembles the proverb, is:
He crushed him with a devastating reply.
Meaning: He defeated him with a decisive argument or a knockout blow.
Similarly:
He shut him down.
This is a common expression for someone who has been completely silenced and has nothing left to say.
Because the proverb's root meaning is a knockout blow that reaches the point of destruction, there is also:
A knockout blow.
Meaning: A blow that ends the matter completely, whether in an argument or a confrontation.

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أي اسكته بداهية أوردها عليه، وإنما قيل بلفظ الجمع لأنهم أرادوا رماه به مرة بعد مرة، ويجوز أن يجمع بما حوله إرادة أن كل جزء منه قحف، كما قالوا: غليظ المشافر، وعظيم المناكب، والقحف: اسم لما يعلو الدماغ من الرأس، ولا يرميه به ما لم يزله عن موضعه وينزعه منه، وهذا كناية عن قتله، فكأنه بلغ به في الإسكات غاية لا وراء لها وهو القتل، والمقتول لا يتكلم.