How did spam evolve into phishing?

As filters started blocking obvious bulk mail attackers shifted to deception. They began sending messages that looked like banks payment processors or internal company mail to trick people into revealing credentials.

The word phishing is a mix of fishing and the older hacker term phreaking which was used for hacking telephone systems. Early hackers used the ph spelling to signal that it came from hacker culture.

It is spam with a disguise. It sails under a false flag and hopes the port will let it in.