The French love to create new words by reversing and/or inverting syllables and letters, a little like pig-latin. This form of slang is called verlan (which itself is a verlan/slang for l’envers which means “reverse”).
Relou comes from the word lourd, which means “heavy or taxing.” The word relou is often used as an interjection similar to “that stinks/sucks” be used in English.
One thing to note: Since certain letters (usually at the end of words) aren’t pronounced in French, sometimes they get dropped in the verlan version of that word. That’s why we see the D in lourd disappear in relou. And, sometimes a a letter might be added (like the E here) to make it pronounceable. Don’t expect verlan to be an exact science … that would be too easy now, wouldn’t it?! 😉