It's an intensification of "what do you want, a cookie?" which shows lack of interest when someone tells something he considers very
cool or
amazing, mostly while showing off whith it.
For example, you can use it when you are down and someone thinks he has to tell you just
NOW how happy and overjoyed he is.
(I am sad because my love is already assigned)
Annoying person: ...and I just kissed her! She requested me to bring her home and in front of her house she asked whether I wanted to see her stamp collction -haha- and of course I said
yes- damn she'
s so friggin'
hot- and then we went in and she took off her...
You:
Hey, do you want a cookie from my shut-up-and-croak-box?