something that tastes soooo much better than just being delicious...soo much better than yum. the word yum wouldn't do this thing justice. Something that sounds soooo good, that when someone tells you about it, all you can say is "yumster".
Also can be used for someone who is beyond hot.
"I ordered a filet today..." "oooh. Yumster!"
"That chic is hot, what do you think" "Damn...yumster"