Barnstorm - The first definition relates to actors. The word originated in the US in the early 19th century when actors actually went about the countryside performing in barns and similar informal venues. Their style was that found in melodramas but it was probably more the makeshift nature of touring which gave it its second definition (used from 1890s) relating to politicians who stomped about seeking votes.
Belt and braces - this applies to a system being used that has a back-up i.e. if one part breaks then another will carry on. The phrase was being used by 1930s by engineers. it was also the name of a British theatre group in the 1970s. An Australian engineer was heard to comment in in 1993 that some of his colleagues would talk of belt, braces and bowyangs too - bowyangs are ties round a workman's trousers to keep out cold and mud.
Berserk - meaning frenziedly mad and deriving from the Berserkers, legendary Norse warriors who fought frenziedly in battle. They wore 'bear-sark' or 'bear-coat' clothing.
Cross the Rubicon - To make a significant decision from which there is no turning back deriving from Julius Caesar's crossing the stream of that name in 49BC passing from Cisalpine Gaul into Italy and so becoming an invader.