Verbalization of '...' which is the written form of a pause in speech.
Verbalizing the written substitue for a short silence adds a level of sarcasm to the comment (or absence there of.) When used in place of a glare to indicate ignorance, it has the effect of softening the regection.
Can also replace the dramatic pause or a pause to indicate confusion.
Girl: Like, OMG! Gambit is soooo Hot!!!
Someone who thinks she's ridiculous but doesn't hate her enough to hurt her feelings: Dotdotdot.
Girl: Oksoiwaslike standingthereandhewasallike 'Whatever' soiwaslike 'What Ever!'
Anyone else: Dotdotdot?
*standing on your desk with your tie around your head, pretending a cardboard tube is a katana*
Smith (jumping out from behind the cubicle wall with nunchucks made from two soda bottles and some sting): Dot dot dot!
You (stiking a super awsome samarai pose): Hwahh!