The affix ṭhak comes in the sense of 'this is whose weapon' after a word denoting 'a weapon' and being in the Nominative 1st-Case in construction.,
Thus असिः प्रहरणमस्य = आसिकः 'a swordsman'. So प्रासिकः, चाक्रिकः, धानुष्कः ॥,
प्रहरणम्
Adhyāya 4 · Pāda 4 · Rule 57