The spelling of the word "amaas" may seem confusing, but it is actually quite straightforward when you consider its pronunciation. In IPA phonetic transcription, it is spelled /əˈmɑːs/. The first syllable is pronounced with a schwa sound, followed by a long "a" sound in the second syllable. The final "s" is silent, making it easy to mistakenly omit it when writing. If you remember to pronounce each syllable clearly, you will have no trouble spelling this word accurately.
Milk-pox.
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.