“Sunshine” is a slang term that we use as a term of endearment to be kind to someone to tell them that they’re a warm and bubbly person. When we say it sarcastically, we’re trying to refer to someone as a bubbly and warm person when it’s clear that they’re not.

(humorous) Used to address someone who has just woken up and/or is very sleepy. Good morning, sunshine! (slang, uncountable, uncommon) Ellipsis of orange sunshine.

(informal) happiness She brought sunshine into our dull lives. (British English, informal) used for addressing somebody in a friendly, or sometimes a rude way

sunshine noun [U] (PLEASURE) informal happiness or pleasure: Their grandchildren have brought sunshine into their lives. mainly UK informal used as a form of address, either in a friendly way, or to express unwillingness to accept another person's delays, bad behavior, etc.: Hello, sunshine! Come on, sunshine, get to work.