User blog comment:Thailog/Ep. 206 "Bloodlines" discussion/@comment-5018693-20120604083438/@comment-69.133.155.229-20120604112752

It means that Wally is an older-first cousin of Bart. Since Wally is Iris' nephew, he is the older cousin of her future childrern (Don and Dawn's first cousin). And since Bart is Iris' grandson, he is Wally's first cousin, once removed.