فُلَانِىٌّ

the rel. n. of فُلَانٌ: it is rendered indeterminate by the affix ى; and by means of the article اَلْ, it becomes determinate; therefore you say فُلَانٌ الفُلَانِىُّ [meaning Such a one, the person named in relation to such a one], (TA,)