The Thampis and Kochammas are the sons and daughters of the maharajahs of Travancore and their non royal or morganatic wives belonging to Nair caste and its sub castes.
Thampis and Thankachis form a part of the Nair caste (even if the mother is a non-Nair, as in the case of the Kunju Thampies) and had no title of succession to the throne. The very term Thampi and Thankachi meant, in Malayalam and Tamil language, brother and sister respectively which indicated the position of the Thampi families as the non-crown inheriting royal relatives of the Royal House of Travancore.
The consort of the ruling Maharajah (King) as well as Elayarajah (Crown Prince) was known as the Ammachi with the title of Panapillai Amma. To the names of the sons of the Maharajahs was prefixed the title of Sri suffixed with Thampi. The daughters were known as Kochammas. The other members as well as the descendants of the Ammaveedus, however, were simply known as Thampi and Thankachi.
Thampi and Thankachi Wikipedia
The Maharajahs of Travancore (current south Kerala) adopted the Matrilineal custom and inheritance prevalent in the land around the 14th Century AD. Accordingly, when a king died, his nephew (sister's son) would become the next ruler, and his own son, born of his Nair wife, would be simply called Thampi with the title " Sri (mother's house name) (personal name) Thampi" which was also one of the highest titles of nobility in Travancore. All of the Maharajahs' daughters were known by the style of Kochamma with the title " (mother's house name) Ammaveetil Srimathi (personal name) Pilla Kochamma". Since the Marumakkathayam system of matrilineal inheritance existed in Travancore, the descendants of these individuals would not gain any distinguishing title other than Thampi (male) and Thankachi (female).
The title of Thampi was also given to some families in Travancore by the Maharajah of Travancore, as a reward for exemplary military, social or government services. Thampis indeed had some special social privileges in Travancore. Besides the Maharajah, they were the only people permitted the use of Palanquins. They also had the right to visit their royal cousins, their father's heir as per the Marumakkathayam law, without formally previously announcing their visit.
According to a noted expert, they formed the third estate colloquially called "Thoruvam Nairs" in the caste hierarchy and their position was next only to the Kiryathil Nairs and Ilathu Nairs