I have four boys! I should be able to come up with something really, really gross. And I did, courtesy of boy #3, Evan, the five year old. It's not gooey or grimey or slimey but it made me gag. And it takes a lot to gag a mother of four boys.
Our family was eating dinner at Steak and Shake with MIL and SIL. Evan was done eating and he was getting restless, as five year olds are known to do. He was wiggling and sliding around in his seat, even sliding so far as to be in the floor under the table. He disappeared under the table and I was glad for the respite. But he never reappeared. I leaned down to see what was keeping him occupied under the table. My adorable little guy had his teeth sunk into a piece of gum on the bottom of the table. He had it stretched out about a foot, with a string of someone else's chewed up gum going from his mouth to the bottom of the table! Ewwwww! Now that's GROSS!