How can "back" be used as verb?

Various phrasal verbs which have got 'back' are:

back somethingup: which means to reverse. Instance of the same are: You'll have to back up your car so that I can get out.

back someoneup: which means tosupport. Instance of the same are: My wife backed me up over my decision to quit my job. My friend has backed me up on convincing my parents to study overseas.