Lekker is not actually slang in Afrikaans. It means nice or great. In South Africa many English speaking people have adopted the word lekker into their vernacular so it has become slang for them.
in afrikaans we just say slang, some of the words we never made so we just say them english. why though? if you're going to talk to an Afrikaaner you kan just say slang they'll understand.
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: rip-off - A bad deal; an unfair price...