Like many other places, the etymology of the Tenom name has many versions. One of them is associated with the Tonom River in Old Tenom.
There is a version that says it's an alteration of the word "Plant". The dialect of the British was not able to properly pronounce the "Tanam" and it became Tenom. The word Tenom existed as early as 1870 from the records of Frank Witti's expedition. The expedition has suggested the construction of a railway to bring Tenom's land out.
Tenom's name is also mentioned in the writings of DD Daly in 1885, a Resident Assistant for the Dent Region. There is also a Tenom name on the North Borneo release map of 1900. A press release on March 8, 1902 explains that the British North Borneo Company (BNBC) changed Tenom's name to Fort Birch. By 1908, the name of Fort Birch was changed to Tenom.
Many associate Tenom's name with the hero Ontoros bin Bundusan. His nickname was Antanom given to him by BNBC after the War of 1915. But it is clear from the record that Tenom's name existed long before Antanom's rise.Source: Tenom District Cultural History and Heritage