It's a relative term.

-"ellam en Machaan paathupaan",
-"Naanum en orepadi'yum dhaan samayal velai ellam gavanichhukarom"
-"Vunga aathu maatupon maadhiri enakkum oruthi kidaichaa nalla irukkum"

We come across many terms for relations (like Machaan, Maatupon, orepadi) in our everyday life. Some of those terms have meanings too, while there are many other terms without any special meaning. In tamil grammar the former terms were called 'Kaaranap peyar' - those which are named with some reason, while the latter are called 'Idukuri peyar' - those terms which were given the name just for identification purposes. Thinking of it, "Isn't 'Idukuri peyar' a 'Kaaranap Peyar' too?".

Here is an attempt to dig deep into what some of those terms mean:

Machaan aka Machinar, Mei thunar - Husband's brother or wife's brother. We all know what 'vuyir' is. It is life, then there is 'mei' which is our body, and the combination of 'vuyir' and 'mei' is a human being. (Maybe, it is from this concept they named the letters of Tamil Language, the 'Vuyir ezhuthukkal', 'Mei Ezhuthukkal' and the marriage of the two- 'vuyirmei ezhuthukkal'). The one whose job specification includes, guarding the body, the 'mei' is called the body guard or 'Mei Thunar' -> Machinar -> Machaan. Your Machaans are supposed to act as your body guard in times of need.

Orepadi aka Oragathi - If A and B are brothers. A's wife is an oragathi or orpadi to B's wife. Long long ago, so long ago, that nobody knows how long ago, there existed a concept called 'joint family'. During those times, all the As and Bs lived under one roof. Since A's wife and B's wife set foot on the same steps ('padi'), they are called 'ore padi's. 'Agam' is another term for house. Since they both live in the same 'agam', they are also called 'ore agathi's.

Naatupon aka Maatupon aka Maatru pen - Daughter-in-law. This is straight forward. 'Maatru pen' means, the alternate daughter. This in due course became 'maatupon' and some also use the term 'naatupon'.

Thambi - Younger brother. Ambi means 'younger boy'. 'Thannudaya Ambi' became 'Thambi'. There are also other archaic terms like 'Vumbi' - 'Vunnudaya Ambi'- your younger brother, 'Nambhi' - 'Nammudaya Ambi' - our younger brother', 'yembi' - 'Ennudaya Ambi' - My younger brother.

Girls, you weren't left out. Thangai - Younger sister. 'Angai' - means 'younger girl'. Similar to the Thambis, etc, we have 'Thannudaya Angai' - 'Thangai', 'Nangai', 'Vungai', 'Engai'.

Ammaan aka Maaman, Maama- Mother's brother. Ammaan comes from 'Ammavin Sagodharan' which later became Maaman, Maama. There was (is ?) also a term called 'Ammaanjee'. 'Sei' means son, 'Ammaan sei' became 'Ammaanjee' i.e Maama's son.

OK, don't ask the etymology of 'Naathanar' - sister-in-law, with all the 'stinking' fight some of them have, I can come up with a story, but it is better to classify 'Naathanar' a 'idukuri peyar'

In those days, when one is not sure of who the father of the kid is, they regarded those kids as God's kid. They called such people "Thevar Adiyavar", today that word took many a twist and turn that we can't use it anymore in a public forum. 'Boss'!, if you are not that 'tardy' you will understand what i am talking about ;)