because clearly you are saying something. usually something pretty nasty. it has an element of bitchiness to it, and kind of feels like you are going to go to the next person you see and slate person 1 until your tongue falls off.
example, a couple of months back I had a disasterous haircut. really really awful. I had my jaw length bob cut into a crop to sort it out. I went round to my dads and DB was there with his GF, who has long blonde hair. and thinks every woman looks better with long blonde hair.
everyone gushed about my new do, and the GF said "Im not saying anything" and left the room.
it made me giggle a bit because it was obvious she didnt like it (and thats not what the thread is about, just an example) but i heard a mum say it today at school re another mum and it frustrated me. I think its up there with "Im not being funny but..."