Why is a Jack called a jack in lawn bowls?

Because it was invented by a man named Jack Bowlington from Jacksontown, in Jacksonville CA.

The above answer is wrong & the person who answered is trying to be a 'comedian' & failing.

There are a couple of theories why the 'Jack' is so called.

1.It comes from the Latin 'jactus' meaning 'to throw or cast'.

2.'jack' in some context means 'smaller version' as in 'Jack-rabbit' IE the smaller version of a rabbit.

'The 'jack bowl' was mentioned in an article by R Pierce in 1697 where he stated..' He cdid not have the strength to throw the Jack-Bowl the length of the green'. Over time the 'Jack-Bowl' was shortened to plain 'Jack'.

There are other theories but these two seem to make the most sense.