According to Cassidy, this was a slang term for a fortified red wine and comes from the Irish fuad, an obscure word for a thief, a wretch or a vagrant. There are two obvious points here – do they make the wine out of vagrants? If not, it’s hard to see why they would call it fuad juice. Secondly, the origin of this expression is pretty obvious. Everyone knows that people when they made wine according to the traditional method used to take their shoes and socks off and tread the grapes until the juice ran out. Isn’t this a much more likely origin for foot juice than an obscure Irish term which sounds a little like foot?