"rum" means "strange" or "odd" so, a common phrase "Tha's a rum'n" means "that's an odd one" i.e. " di' yew see Ted down the pub?" "wh' noo, oi din't" "wh', Tha's rum'n, he's allus in there" ("did you see ted in the pub?", "well no, i didn't" "well, that's odd, he's always in there" )
so, "He's a rum boy" means "he's an odd boy"
christ, where did you get that pic of my cousins wife?! seriously, she's about the rankest thing i've ever seen! her daughter (the girl i'm moving in with in a couple of weeks) is so embarrassed by her mother that when she's out, she always refers to my mu