Kat ~ didn't mean they would hang the washing on the guns; you miss the point of the joke ~ not surprising, because it is UK-idiomatic. In this country, the string stretched between two poles on which damp washing is pegged out to dry ['hung out'] is called a 'clothes-line': hence "Hang out the washing on the Siegfried Line".

The words of the song BTW were by the ubiquitous Jimmy Kennedy [Red Sails In The Sunset, South Of The Border, Isle Of Capri, Teddy Bears Picnic, The Hokey Cokey...]

~Michael~

From Wikipedia ~ "We're Going to Hang out the Washing on the Siegfried Line" is a popular song written by Ulster songwriter Jimmy Kennedy, whilst he was a Captain in the British Expeditionary Force during the early stages of the Second World War. The Siegfried Line was a chain of fortifications along Germany's Western border, analogous to the Maginot Line in France. The song was used as a morale-booster during the war, particularly up to and during the Battle of France.
It began
We're going to hang out the washing on the Siegfried Line.
Have you any dirty washing, mother dear?
We're gonna hang out the washing on the Siegfried Line.
'Cause the washing day is here.