Do I take it, then, Tim, that "mug" is not used by Americans in the sense of "someone easily deceived"?
Mug meaning ‘gullible nitwit’ is more a Britishism than an Americanism. In old gangster patois, I suppose the equivalent term was ‘sap’, but you don’t hear that much anymore. It was certainly a common usage back in the early 20th Century.
Here, from addled memory, are the first verse and chorus of one of my favorite English music hall songs, "Trinity Church", from the first decade of that benighted century:
(In 2/4 time)
"Twelve months ago, with decent chances,
Prospects of success in life,
Through foolish love and ballroom dances,
Troubles came: I met my wife.
Such a charming, lovely creature,
She in my eyes did appear,
False, she was, though fair of feature,
Like the salmon, I was speared.
Chorus (in 6/8 time)[/]
She told me her age was five-and-twenty,
Cash in the bank, of course, she’d plenty
I, like a lamb, believed it all,
I was an M-U-G.
At Trinity Church I met my doom,
Now we live in the top back room,
Up to me eyes in debt for rentee,
That’s what she’s done for me."
- PW
there, that’s better