It's なさる (honorific equivalent of する), nor なす. Other verbs that conjugate this way (aる->aい) include 仰る, いらっしゃる and 下さる (honorifics of 言う, 行く/来る/いる and くれる, respectively). The naked -masu stem of these verbs is also an imperative, hence -なさい and -てください.
Anyway, appending なさる wouldn't normally add anything to the meaning, but rude + honorific -> sarcasm.
Last edited by Vempele (June 02, 1:22 pm)