Someone or something unexpected.
"The sudden arrival of our old friend was completely unexpected."
"The unexpected was his sudden arrival at the party, which surprised everyone there."
Not expected, anticipated or foreseen.
"The sudden rainstorm was completely unexpected, leaving us soaked without an umbrella."
In plain English: Unexpected means something happens when you did not plan for it to happen at all.
"The surprise party was unexpected, so everyone jumped when they walked through the door."
The word unexpected is formed by adding the prefix un- to expected, meaning it arrived into English as a straightforward negation of something that was anticipated. Its usage simply reflects the concept of an event occurring contrary to what was predicted or hoped for.