means As a noun, a piece of mail, a fixed upright support, an assigned job or station, or a published item online; as a verb, to send, station, or publish.

from This is really several words wearing one coat. The 'upright pole' sense comes from Latin 'postis,' a doorpost or support. The 'mail' and 'station' senses trace to Latin 'positus,' meaning 'placed,' via the relay stations where horses were stationed along a route — to 'post' a letter was to hand it into that relay system, and to be 'posted' somewhere was to be placed there. The 'job' sense grows from that same idea of an assigned position. The online sense is the youngest, borrowing 'post' as in 'to make public' — itself an extension of pinning a notice to a post for all to read.