According to 中華民國教育部國語辭典, 麻 in 芝麻 reads with a 轻声 but with a rising tone in 胡麻, 脂麻 and 油麻, and the funny thing is that they all mean the same, i.e., “sesame”. The reason behind the tonal difference, I guess, is that 芝麻 is a transliteration (音译) while 胡麻, 脂麻 and 油麻 are translations (意译). 芝麻, 原称胡麻 (学名: Sesamum indicum), 相传是西汉张骞通西域时引进中国的. The Chinese word 胡 classifies it as a foreign product and its 学名 “indicum” points out an Indian origin. Sesame is called tila in Sanskrit, from which tel, the Hindi word for “oil” (油 or 脂), is derived. Unlike its synonyms, 芝麻 seems to have a different etymology. 芝 usually refers to a type of fungus, but in 芝麻, it has no real meaning. It simply transcribes a foreign sound (音译). I believe 芝麻(Middle Chinese *ćɨma) was borrowed from Aramaic -šəma (Aramaic šumšəma “sesame”) through Central Asian tribes (e.g., 大食帝国). 麻 in this case represents an unstressed syllable of a foreign word and therefore is pronounced with a 轻声. My personal opinion: The tonal distinction between 芝麻 and its synonyms (胡麻, 脂麻 and 油麻) lacks functional significance. I prefer zhi1 ma2 (rising tone) and think it is as “proper” as zhi1 ma (轻声). What do you think?