Tight up means "apertado, junto". So, in this part of that song they are tryna say that "their hands are touching". In Portuguese, "as mãos estão apertadas".
Sit tight I got it like "sit down carefully" and the another word I've look up and find that means something where you put your book on top.
Spend/spent means loosing time doing something that you don't have or need to. So, in this case, they are trying to say " look at all this time we've already lose with love"