By form a metathesis away from Coptic ⲉⲥⲑⲏⲙ (esthēm), though derivation from an unattested oblique δ (d) form of Ancient Greek στίμμι (stímmi) has been suggested, ultimately from Egyptian sdm, possibly also conflated with Egyptian msdmt.
Practice speaking and memorizing "إثمد" and many other words and sentences in Arabic.