In Chinese, plural forms are almost non-existent. The name of the song is 瓜老外, in which 瓜 means stupid, 老外 refers to foreigner (or foreignerS, see? that’s what makes things tricky). However, in this context, “瓜老外” refers to a specific type of foreigner, rather than referring to all foreigners in general.

Melon means stupid, in Mandarin it would be 傻瓜shagua meaning stupid melon. Xiedi is speaking in Sichuan dialect