Where Did That Come From?

Armchair Expert  - an armchair expert is someone who knows a lot about something from reading about it (in an armchair presumably) but has little or no real world experience.  For instance if someone had studied up the road code they may not necessarily be able to drive.

There is also a US expression – armchair quarterback.

Lame Duck – this means a person who is at the end of their time in a position or office who is thought to have less power and influence as a result.  But why a duck?  Apparently the first use of the term was in relation to stockbrokers who couldn't meet their financial obligations.  I guess that made them lame and liable to be attacked by predators.

Beat around the bush – It is thought that this originated in game hunting where beaters would be used to scare birds into the air for hunters to shoot at.  But it is a stretch to get from there to its modern meaning – which is someone talking around a subject without getting to the point.

Let one's hair down – in olden times women were expected to have their hair long and pinned up in elaborate hairstyles.  The only time they could relax was in their own boudoir.  So the phrase became associated with a time where you could enjoy yourself without worrying about conventions or etiquette.