when you watching a cartoon you haven't seen since you were seven and you start having flashbacks of those times or just thinking about when you were younger
"Bruh this episode of SpongeBob is giving me a nostalgic high, I haven't seen it in so long"
A hardo is a person who tries extremely hard at everything. Most times a hardo will try very hard at things that do not require excessive effort. For example, sports practices, gym class, etc.
Wow did you see Jack at practice today? What a hardo!