From Wiktionary, the free dictionary
amui + Cantonese 仔 (zai2, “diminutive suffix”) and/or Cantonese 阿 (aa3, “familiar prefix”) + Cantonese 妹仔 (mui6-1 zai2, “maid”).
amuichai
- diminutive of amui: very young Chinese girl
- young Chinese servant or maid