From 滅多(めった) (metta) + 刺(さ)し (sashi, “連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 刺(さ)す (sasu).”). The sashi changes to zashi as an instance of rendaku (連濁).
めったざし or メッタざし • (mettazashi)