Although the term has historically had some derogatory connotations, it has entered common parlance as a neutral term in Singapore and Malaysia, where it refers to a white person or, when used as an adjective, Western culture in general.
The earliest origin for the term ang mo could be traced to the contact between Hokkien (Southern Min) speakers in southern Fujian with the Portuguese Empire and Dutch East India Company during the Haijin ("Sea Ban") period in the 16th and 17th centuries.
Dutch people were known in Taiwan as ang mo lang ("red-haired people") in Taiwanese Hokkien.
The Chinese characters for ang mo are the same as those in the historical Japanese term kōmō, which was used during the Edo period (1603 - 1868) as an epithet for (northwestern European) white people. It primarily referred to Dutch traders who were the only Europeans allowed to trade with Japan during the Sakoku, its 200-year period of isolation.
The 'red haired' traders of the Dutch East India Company were confined to Dejima, an artificial island abutting Nagasaki. It was to become the injection point for Western technology, science and medicine which, for over two hundred years, infused Japan with the knowledge and methods that would serve her well.
"Dutch" + "Learning" = Rangaku
The first Dutch surgeon (he was actually German) who had significant influence on Japanese medicine was Carl Schamberger, from Leipzig (1623 - 1706). He arrived at Dejima in Nagasaki in 1649. He soon came to the attention of the Imperial commissioner Inoue Masashige, who had already embraced Western thought. Before long Schamberger acquired as patients a number of high ranking members of Japanese society . At the end of his two years in Japan, the governor of Nagasaki commanded a report on his methods which became the basis for the first Western practice of medicine in Japan known as Caspar-style-surgery, or kasuparu-ryû geka.
Another important surgeon was Hans Juriaen Hancko, from Breslau, who stayed in Dejima from 1655 to 1658. In Nagasaki he taught Mukai Gensho (1609 - 1677) about oinments, medical oils and medical herbs. Mukai's record is preserved as Komo geka hiyo (Secret Essence of Red-Haired Surgery).
Other western doctors operating in Dejima were the German Engelbert Kaempfer and the Swede Carl Peter Thunberg, a pupil of Linnaeus. Unfortunately, we couldn't identify which one among these surgeons actually had red hair.
For more information see;
Rangaku: "Red Haired" Medicine In Shogunate Japan
The History of Ophthalmology in Japan
The Netherlands East Indies and Japan