-=Original (wrong) answer=- It comes from a song.... called "Johnny Come Lately" (by Steve Earl and the Pogues) It's about a man named Johnny who's never there when he needs him, so they dub him Johnny-come-lately. It's someone who's gone when you need them but are around when it's conveinient, typically referring to someone you love. They're a knight in shining armor who always shows up too late. -=Edit=- Ok, I'll be the first to admit that I was wrong. I've heard the term used alot with Polari (gay slang), and I heard the song only vaguely on the Muzak at work. So this is what I thought it meant. But today I heard the song again and have reconsidered my answer. A Johnny-come-lately is someone who is new (in town, at work, etc), simple as that. However, it is more commonly used when said person is very successful and well-liked, often used negativly by those who would be considered "jealous" having been there longer and not had the success as Johnny-come-lately. It can refer to male or female. Also, although Steve Earl and the Progues wrote a song called "Johnny Come Lately" (which wasn't the song I thought it was O.o), the real source of the word is much earlier. It was the title of a 1943 movie, and was found in a 1839 publication (which read "She may be a Johnny-come-lately on the board, but she's doing a fine job with publicity.") This is the oldest known source of the term and is said to come from Great Britain or Ireland. It's origin is shady but I hope this helps.

SYLLABICATION: John·ny-come-late·ly NOUN: Inflected forms: pl. John·ny-come-late·lies or John·nies-come-late·ly (jnz-) Informal: A newcomer or latecomer, especially a recent adherent to a cause or trend. i.e., You could use this at work I suppose..."The new Johnny-come-lately will mop the floor." Meaning the new guy/girl. Kinda like lowest person on the totem pole phrase.