I've heard Fregly's a really smart kid, but anyone who sits out in their front yard and packs dirt in their ears can't be all that sharp.
More context:
http://www.funbrain.com/journal/Journal.html?ThisMonth=1&ThisDay=4&ThisJournalDay=118&ThisPage
Given the kind of boy Fregly is, maybe you can take it literally: one of his occupations in actually putting dirt into his ears... If it's an idiom, I've never heard it.
Thank you, Nona the brit, Pieanne !