Definition of back:

part of speech: adjective

That is situated behind; previous.

part of speech: noun

A brewers vat or large open tub for containing beer; a ferry- boat.

part of speech: adverb

To the place from whence one came; to a former state or condition; behind; not advancing again.

part of speech: noun

The upper part in animals, and the hinder part in man; the rear; the part out of sight; a miners term for joints; that part of a mineral lode nearest the surface.

part of speech: verb

To mount; to support; to put or move back.