Acronym for "Yeah I'll be there". Commitment to be present at an event, set for a specific time and place, expressed with an extreme lack of sincerity intended to be understood with sense of hipster irony.
X: "Hey Y, are you going to be at my wedding next year?"
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!