From Arabic ثَلْج (ṯalj), from Proto-Semitic *ṯalg-. Maltese s- (instead of expected t-) may be due to irregular assimilation of [θ] to the following alveolar [l]. Otherwise it could be speculated that the form goes back to *šalj with š- from a Semitic substrate (as perhaps in xemx). The subsequent dissimilation š- → s- under the influence of following -j- would be less surprising (compare e.g. siġra).