£2.50 Two Pounds and Fifty pence in modern parlance. In 'old money' 2/6 two shillings and sixpence = twelve-and-a-half 'p', but with the rise of inflation, half a crown has more of a resonance with £2.50p
Hey Malcolm, can I have half a crown to cover your entry money for next year's Middlesex League?
The meaning of this term defines a state where everyone in the world is gradually being sent information about a website (or other) in an amazing rollercoaster effect. One person sends a weblink or idea to someone else. This second person sends it to ten people. The next ten people send it to a hundred people etc. etc. This is the blue wave effect.
Someone sent me an amazing video last week. This week I've had it via email from three other sources. Must be on a real blue wave there!