A "Yeahmate" is a posh boy, who acts like a posh knob. Commonly seen wearing Jack Wills, Abercrombie and Fitch, Polo with his jeans below the standard position of a boy or man. His reply to most questions end in mate. E.G "Mate did you see the ass on that yat?" "Yeah mate, it was sweeeet mate"
Donovan "Oi, look at that yeahmate"
Marco "He's a right twat. Proper yeahmate and all"