Used to show lack of concern, respect, and also to show anger, hatred, and jealousy. It is often stressed for more impact to provoke the listener.
Most people mistakes it for 'sighing' but it is rather called 'Hissing'
Example:
A: No be your ex be that
B: Mtchew! Leave tha Ashawo!
(A: Is that not your ex?
B: Mtchew! Let me with that whore)
A: Young lady can't you greet?
B: Mtchew! Na u dey give me chop?
(Mtchew! Are the one feeding me?)
