"Up the Reik without a barge" - In deep trouble, often abbreviated "Up the Reik."
"Short crowd" - A gathering of angry people with no power. They don't pose a threat, much like a bunch of halflings wouldn't (they would make up a short crowd, hence the expression).
"Carstein Cattle" - inhabitants of Sylvania.
"Drunk as a map-maker" - derived from the fact that many maps of the Empire often are faulty and inconsistent, hence they are probable made by drunken map-makers.
"Moose-head" - norse raider. Refers to the helmets often worn by the Norscan warriors.
"Tybalt handshake" - a weak handshake, derived from the fact that chancellor Mornan Tybalt had his thumb clipped by rioters during a riot some years ago, in protest of his proposed labour tax.
"Altdorf party" - a riot.
"He's riding the griffon" - someone is boasting, greatly exaggerating his prowess (often "romantic" prowess or combat skills). Could also mean that a person is mad, or hallucinating. Derived from the legends about The Emperor riding a griffon.
"Mad as Alfred" - Describes a person out of his mind, insane or just very, very different. Derived from Altdorf's most famous and loved madman, called Mad Alfred. I'll give you more details on this fascinating person in later writings.
"There's no such thing as a free pie" - nothing in the Old World is free, everything has its price. You'll never get anything for nothing. Also "a deal to good to be true".
"A private war" - someone's own personal business. "Well, it's his private war, there's nothing we can do to help".
"Airhead" or "Cloudhead" - derisive term for Wizard, implying that they have their heads stuck in the clouds and consider themselves above mere mortals. Not used when a wizard is around, I gather.
"A Bergsburg deal" - a deal or offering that is on and off, depending on the whims of the people making the offering. Refers to the disputed status of the town of Bergsburg among the Imperial cartographers.
There are many more sayings like this, and I hope to document more as I spend more time talking to the people of Altdorf.
Konistag, Sigmarzeit 18, 2522 IC
Check out this link to the Black Industries forums, where Happy Gibbon started a fantastic thread about the sayings of the people of the Empire. The above list is my contribution, and you'll find many more from Happy Gibbon as well as other contributors.