The origin of the name Bangkok (บางกอก, pronounced in Thai as [bāːŋ kɔ̀ːk] (listen)), is unclear. Bang บาง is a Thai word meaning 'a village on a stream',[13] and the name might have been derived from Bang Ko (บางเกาะ), ko เกาะ meaning 'island', stemming from the city's watery landscape.[9] Another theory suggests that it is shortened from Bang Makok (บางมะกอก), makok มะกอก being the name of Elaeocarpus hygrophilus, a plant bearing olive-like fruit.[d] This is supported by the former name of Wat Arun, a historic temple in the area, that used to be called Wat Makok.[14]
Officially, the town was known as Thonburi Si Mahasamut (ธนบุรีศรีมหาสมุทร, from Pali and Sanskrit, literally 'city of treasures gracing the ocean') or Thonburi, according to the Ayutthaya Chronicles.[15]Bangkok was likely a colloquial name, albeit one widely adopted by foreign visitors, who continued to use it to refer to the city even after the new capital's establishment.
source: wiki