One of them – ‘to be in up in arms’ – is cheating slightly, as it’s not really about ‘arms’ the parts of the body, but ‘arms’ in the sense of weapons. Originally, being ‘up in arms’ about something meant being so angry that you had your your weapon up, ready to fight.
‘Armchair general’ is particularly appropriate right now, in the middle of the European Cup football matches. Football fans and would-be experts are sitting in their armchairs, shouting at their TV screens or posting on Twitter and telling the managers of the national teams how to do their jobs! I wouldn’t like to be Gareth Southgate right now! An expression with a similar meaning is ‘back-seat driver’ – someone who’s not in charge, but is happy to tell you what to do.