Tam-is literally means sweet in the melodic Hiligaynon language. It is also what Ilonggos call a variant of a tuber-crop related to the ube, potato, taro and yam, among others, with a collectibe? scientific name of Dioscorea esculenta, syn.: D. fasciculata. Tubers are the enlarged storage tips of a rhizome. The “eyes” are actually buds in nodes, arranged in spiral pattern from the base to the apex of the tuber. Aerial tubers, which are common in yams, are called tubercle. (Source)
So when most people have their first bite of the tam-is, they find it weird and ask why isn't it sweet. Now I don't have the "etymology" (how the hell it got its name) for tam-is, but it brings back the childhood memories for most especially those with access to rural goods. Tam-is is starchy but its "smoother" compared to kamote and cassava. One can find them during market days especially in inland towns where farming is abundant and crops come in different kinds. Nowadays, tam-is can be considered as a novelty for its not as popular but it's name will certainly ring a bell or may "elicit" curiosity.