This preposition (or the “exceptive particle” as it is usually referred to in Arabic grammars, see Wright 1974 II 335) probably goes back to PS
*ˀila/*ˀilla. There are, however, some arguments against such a connection. Akkadian forms
ela,
elat,
illa are thought to be derived from the adjective elû ‘high’ (”above” > “besides”, see GAG 206, Tomback 1978:248), while Arb. ˀillā is usually analyzed as a combination of ˀin ‘if’ and lā ‘not, no’. See Arakelova 2001:28.