Delulu is short for delusional. Commonly used when someone is extremely blind to signs of their crush not liking them. It is also used a lot to describe a person who is in love with an idol/celebrity.
A phrase coined from the term "delusional". It first originated in the K-pop fandom, but spread more mainstream. Most people use this term to describe themselves or others if they catch them simping over someone.
Did you see Brody staring at Julie in classtoday? He's totally delulu.