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1.
1. The annoyed prolonging sound you make without opening your mouth much with gritted teeth, when a herculean task is thrown at you and you're too lazy to do it.

2. It can also be used in place of an oppressed sigh sound.

3. It can also replace a 'nod of disapproval' at times in a sarcastic way.
Jay : Hey man, can you take care of my assignments for the day, I gotta visit my sick granma.

Sid : *plch* I have too much on my plate already, but I'll try to manage.
by Lady Ghagra October 04, 2011