1. suddenly (appear, stick out, etc.); abruptly (happen); unexpectedly; looming (e.g. out of the dark) (Onomatopoeia, ぬうっと is less abrupt)

2. (stand) still and expressionless; imposingly; menacingly (Onomatopoeia)

3. suddenly (stand up); abruptly (rise) (Onomatopoeia)