The Vietnamese word "kền" has a couple of meanings and uses, depending on the context.
Nickel-coated: In a more technical sense, "kền" refers to something that is plated or coated with nickel. For example, you might see it used when describing bicycle rims:
Crack or swell: Informally, "kền" can also describe someone or something that is very good or excellent, especially in sports.
In summary, "kền" can refer to something that is nickel-plated, or it can be used informally to describe someone who is excellent at what they do.