Used as a farewell, the phrase "smell you later" is another way of saying goodbye. Invented in the 20th century, It's prime usage was in the 1990's. It's slow decline started in the early 2000's. The phrase was mostly used by young ages. "smell you later" is very similar to "see you later" and means the exact same thing. Although not as popular as a farewell anymore, people still recognize the phrase and it's meaning.
I have to go, I can't be late to work. I'll smell you later.