Borrowed from Arabic.
Throughout MSA, *t- in the reflexes of PS *tišˁ- ‘nine’ is lost, whereas *š is reflected as *s (see Testen 1998a and SED I p. xci. for the affricate theory of PS consonantism). Admittedly, forms with t- (and ‑s‑) can be found in the Vienna corpus (Bittner 1916:50).
Borrowed from Arabic.