Not well-known.
Dark, faint or indistinct.
Difficult to understand.
Hidden, out of sight or inconspicuous.
Unknown or uncertain; unclear.
To hide, put out of sight etc.
To render obscure; to darken; to make dim; to keep in the dark; to hide; to make less visible, intelligible, legible, glorious, beautiful, or illustrious.
Not known; unidentified; not well known.
A variable (usually x, y or z) whose value is to be found.
A person of no identity; a nonentity
Any thing, place, or situation about which nothing is known; an unknown fact or piece of information.