Argued to be a loan from Aramaic שׂוּרָא / ܫܘܪܐ (šūrā), but other cognates such as Sabaean 𐩣𐩬𐩪𐩥𐩧𐩩 (mns¹wrt, “walls”) and Tigre ሶር (sor, “partition wall”) (unless that form is itself borrowed from Arabic) suggest that it may in fact be inherited.
Practice speaking and memorizing "سور" and many other words and sentences in Arabic.