what does johnny come lately mean?

Answer 1

It means a newcomer of any kind, usually in a derogatory tone.

A British expression from the early 1800s is "Johnny Newcome," applied to a sailor newly assigned to a naval ship. Johhny-come-lately is an apparent Americanized version that also appeared in the 19th century. Charles F. Briggs' 1839 novel The Adventures of Harry Franco is thought to be the first appearance: "'But it's Johhny Comelately, ain't it, you?' said a young mizzen topman."

Answer 2

Informal A newcomer or latecomer, especially a recent adherent to a cause or trend.