1.Definition: being or having an unknown or unnamed source

"A poem by an unknown author"

Related Adjective(s):nameless, unidentified, unnamed

2.Definition: not famous or acclaimed

Related Adjective(s):obscure, unsung

3.Definition: not known

"An unknown amount", "An unknown island", "An unknown writer", "An unknown source"

4.Definition: not known before

"Don't let anyone unknown into the house"

Related Adjective(s):strange

5.Definition: not known to exist

"Things obscurely felt surged up from unknown depths"