Where they come from

Sport, sea, war and work.

Common sources

Sailing gives learn the ropes and taken aback. Sport gives the ball is in your court and a level playing field. Warfare gives bite the bullet and hold the fort.

Origins are frequently disputed and the popular story is often invented.
Shakespeare and the Bible supplied a large share of the older stock.

New ones

Idioms keep forming: throw under the bus, move the goalposts, low-hanging fruit. Recent office idioms date faster than old ones.