A quick web dictionary search gave this: [L. arsenicum, Gr. 'arseniko`n, 'arreniko`n, yellow orpiment, perh. fr. 'arseniko`s or better Attic 'arreniko`s masculine, 'a`rrhn male, on account of its strength, or fr. Per. zern[=i]kh: cf. F. arsenic.]

A website on the history of arsenic says it might come from the arabic Al zharnick "gold colored" but I think the stuff is so old, going way back to Greek and Persian times hundreds of years BC, that nobody really knows.