Is the thumb a finger or its own classification of appendage? My friends say its a finger, my science teachers say its its own appendage, I just want to know so I can rub it in someone’s face. 
It can be either, depending on how you use the word. There’s no doubt that the thumb is a digit, though (but so is a toe). The SOED defines finger as “Each of the five terminal members of the hand, or each of the four, excluding the thumb.” The Harcourt science dictionary defines it as “any of the five digits of the hand, each consisting of a metacarpal bone and three bony phalanges”
Medically the thumb is a type of finger. It is opposible but not in all animals.
Still one could argue the point.
For example I say that Oscar is a GROUCH NOT A MONSTER.
Some people say a grouch is a sub species of monster. I say it is totally different.
I guess you would have to look at the bone and tendon make up of the thumb. First of all the other fingers bend 3 times. The thumb only once.
Yes it is. 