wouldn't know him from a bar soap

[jeninem]

what is the origin of this saying?

[Quizmonster]

The 'standard' version of this is "wouldn't know him from Adam" but there are loads of variants...the rest of us...Adam's cat...a fire plug...an old sow...rancid owl spit...a hole in the ground...and so forth. 'A bar of soap' is just another of those. I imagine you're as likely to discover who originated it as you are to discover the source of any of the others. Basically, you can make these up at will. How about 'I wouldn't know him from a bat's ear'?

[Octavius]

I wouldn't know QM from a gorbellied urchin-snouted moldwarp.

[Quizmonster]

Actually, O, I look rather like one of those, so you probably would know me if you saw me!