Keris as we know refers to a short dagger from South East Asia. It has decorative handle and a wavy blade reminiscent of a snake. It is said to have originated in Indonesia.
I had a hunch that the word keris could have originated from Tamil. The reason is as follows:
Now, Echis carinata, is a short Indian viper that is native to the Indian Subcontinent. It is about 40 cm in length. It is nocturnal and inconspicuous. Because of this and the speed of its strike, and its readiness to bite at the smallest provocation make this one of the most dangerous reptiles of India. This snake moves about mainly by sidewinding: a method at which they are considerably proficient and alarmingly quick.
Source of above drawing:
Moreover, it is common knowledge that snake occupies an important in Indian cultural tradition.The Tamil name of the snake is kuruncooli (குறுஞ்சூலி) that is made of two words "kuru" + "sooli". Now, kuru means short in Tamil. Sooli means a holder of "sool". In Indian culture, "sool" denotes the trident of Lord Shiva. Therefore the snake appears to have been named after the trident of Lord Shiva.
One of Lord Shiva's numerous names is Lord Pasupathi, meaning the Lord of All Creatures. He wears a garland of snakes indicating that even the feared snakes and dragons are under his control. It means that no harm comes to the person who worships Lord Shiva with deep devotion.
This short dagger so reminiscent of the Indian viper, could have been nick-named as kuru-sooli also. There is evidence that swords and daggers with wavy blades were used in ancient India. For example look at the above picture showing Lord Indra, the King of Heavens in Hindu mythology, riding his white elephant Irrawadhi and holding two 'keris'es in his hand.
When Indian traders came to South East Asia more than a thousand years ago, long before Islam, they could have brought along these little daggers to Java where they gained popularity and spread throughout South Asian countires.
Before Islam came into South East Asia, the Javanese were Hindus and Buddhists. Their metal workers caste known as pande had taken these short daggers to new heights giving them several new designs where they had become more of a ceremonial and religious artifact rather than being used as a weapon. They came to be called keris in Indonesia and in South East Asia.
I surmise that keris is the corruption of the Tamil word kuru-sooli by getting shortened from kuru-sooli to 'kurusu' and then to keris. Since Malay did not seem to have a written form until the arrival of Indians in the ancient days, the chances of oral corruption of Indian words when in used among Javanese was high. Indian words abound Malay language even today. For e.g, the expression "bahasa jiwa bangsa", agama, mangga, permaisuri (parameswari), singgasana (singga asanam) and dirgahayu (dirga ayul) etc. Incidentally, the word agama in Sanskrit refers to Hindu religious texts but somehow came to represent religion in general in Malay.
Hence it will not be any surprise if kuru-suli had become keris over the years. Perhaps some ancient Javanese books written in Sanskrit writing which was then popular and used in both North and South India also, might hold clue to this.
An alternative concept could be that the word keris could have been derived from the Tamil word "Kirisu". Kirisu means Lord Shiva in his form as the inhabitant of Mount Kailasa, located in today's Tibet. The Hindu Javanese believed that the powers of Lord Shiva came to be vested in the keris.
Researchers have said that keris was initially used by the Hindu nobles of Indonesia. However, over the years keris became a symbol of inspiration,valour and spiritual strength to even the ordinary Hindu Indonesian in his or quest to strengthen resolve against Islamic colonisation which was sweeping though Indonesia. Further down the road, even the Muslims of Indonesia had come to preserve the keris as part of their traditional culture. It came to accepted as part of Malay culture.
Read the following excerpt from Draeger's book:
"On arrival of Islam (in Indonesia), the Hindu warlords and their pande scattered, but their old faith in Vishnu and Civa did not die. The powers of these two Hindu gods came to be passed into the keris, and the wearing of the keris now shifted from the exclusive right of the noble class to a universal practice whereby every commoner carried one. The keris took on a new significance and its reputation grew as the power of Civa was invoked for anti-Muslim endeavours. By the commoner wearing the keris , design features were affected; it was made to be worn comfortably and hidden easily in the sarong, ready for instant use. On the fall of Majapahit, the pande were driven into East Java, Bali, Madura and the Celebes, there to fashion keris with still further modifications according to the needs of each geographical area. The original keris forms, said by Ngabehi Karjadikrama to be only four (Brodjol, Tilamputih, Sangkelat, and Panimbal) from which all others derive, were greatly diversified." (Source: Donn F Draeger , 1992, “The weapons and fighting arts of Indonesia”, p.94, Tuttle Publishing, North Clarendon, USA )