appearing and disappearing at will; elusive; phantom-like
Describes someone or something that can come and go freely, often with a mysterious or untraceable quality. Used for ninja, spies, ghosts, or elusive people.
Ninjas move about, appearing and disappearing at will.
He is elusive and hard to catch.
Similar meaning, but 神出鬼没 is more common and often used for someone who appears unexpectedly in various places.
Yojijukugo composed of 出没 (appearing and disappearing) and 自在 (freely, at will). The exact historical derivation is uncertain.