Historically, the character 鞑 was often used to refer to northern nomadic peoples.
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history
鞑 appears in classical Chinese texts as part of 鞑靼 (Dádá - Tatars/Tartars), a historical name for various Turkic and Mongolic peoples of the northern steppes. The term 鞑虏 was used as a derogatory label for Manchus during the late Qing period.
Leather on the left fits the historical context: nomadic Tatar peoples were associated with horse-riding, hide armour and leather goods. The radical lends a faint cultural overtone, though here it is mainly the formal carrier for the foreign-name phonetic on the right.
Supplies the sound dá unchanged. 达 is recruited purely to transcribe the foreign syllable Tat- into Chinese, in the same way 蒙 carries Mongol. The reach meaning has no bearing on the ethnonym.
