Unknown or uncertain; unclear.
Dark, faint or indistinct.
Difficult to understand.
Not well-known.
Hidden, out of sight or inconspicuous.
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.
That cannot be ascertained; uncertain, unable to be determined or discovered.