Bent as a nine Bob note

I know what it means bent=crooked a nine Bob note never existed it was a ten Bob note which is really a ten shilling for some reason a shilling was know as a Bob so if u had a nine Bob note it was counterfit so it was crooked or bent this phrases was used to express a fear of something being illegal

that all I know would like more input thanks

There's an expression in the U.S. -- queer as a two-dollar bill. Meaning there is something "off" or wrong. This sounds similar to the above.

A $2 bill was actually issued. See -- www.treas.gov/education/faq/currency/denominations.shtml#q5

Anyway, I consulted a reference and it said the U.S. expression is "queer as a $3 bill." But I stand my ground and say it's $2.

I looked for bent as a Bob in my two British expression books and couldn't find anything.

ESC, I really think it's "queer (or phony) as a $3 bill". As you indicated, it's commonly known that a $2 bill exists, even though they were never in wide circulation.