How is slang word become word?

Answer 1

Usage. In 1970 the word ripoff was two words and was considered inner city slang. Forty years later it's one word (usually hyphenated) and has a formal meaning:

1. rip-off - A bad deal; an unfair price or rate; A theft or robbery; A scam; A copy, especially one that is illegal or inferior.

It obviously had a very specific meaning that did not originally have a word or phrase assigned to it in English, so when the slang version came along it stuck.

Groovy? Not so much....