A toothless wonder is an individual in the public who is lacking a single front tooth. This person obvious doesn't care about their dental appearance because if they did, they would have stayed hidden indoors and taken care of this problem long before anyone saw it. Also, when you talk to these people, if you make cracks about dental issues, they don't realize you're talking about them!
"Hey dude, did you see that toothless wonder walking by?" "Naah, man, that's not a toothless wonder, he just got the shat kicked outta him, that's all!"