ܙܪܕܫܬܩܢܐ zrdštqnʾ (plur.) Zoroastrians; followers of Mazdak, son of Bāmdād, disciple of Zarādušt, son of Khōragān (Nöldeke 1879, 445–467)
As Gignoux proposes (personal communication), the Syr. form represents MP Zardušt + suffix -akān, that is to say Early MP Zarduštakān, later Zarduštagān, see ManMP zrdrwštgʾn /zardruštagān/ Zoroastrian (Durkin-Meisterernst 2004, 384) ● J St 16, 21 ◆ LS 206a; PS Suppl. 116
PS Suppl. suggests that the Syr. word is a rendering of a Persian patronymic from Pers. Zardušt + -ān; this interpretation is clearly wrong.