The surname ANGCO is one of the most prominent Chinese Filipino surnames in Ilocos Sur history. Although most Chinese Filipino last names remain the same - one syllable like Tan, Go, Lim, etc. - those who became prominent and tried assimilation into mainstream Filipino and mestizo societies during the Spanish period also more or less tried to Hispanize their last names. Thus, we have surnames such as Teehankee, Angbuete, Syquia, Lacson, and more as a clear sign of the merging of a Chinese ancestor's full name into one single name.
The surname Angco, then, clearly means a combination of ANG and CO. Although both ANG and CO are valid Chinese last names, there was also another peculiarity in Chinese Filipino naming patterns during the Spanish period. Typically, when a certain Chinese individual reached a certain level of success or prominence in society he was allowed to attach the suffix "-co" to his last name. In the Spanish period, Chinese individuals who had the syllable -co at the end of their last name (like Cojuangco, Gandiongco, etc.) were clearly men of certain stature, with the suffix equivalent to the title of "Don" among the Spanish and mestizo population.
Thus, we can conclude that the surname ANGCO is a variant of the single syllable Chinese last name ANG (吳), which in turn is a variation of the last name Wu, which means "god-like" or "the highest".
For further history see Surname Spotlight: GO.