Short for "I Want Your Life". Usually said to a person who lives an amazing lifestyle when your own life sucks. Said normally to rich people, or girls with hot boyfriends. Mainly people who have it all/everything.
Girl 1: "I'm going to California for 2 weeks, then when I get back I'm getting a pet chihuahua!"
Girl 2: "oh my god IWYL".
Its an abbreviation that means "I'm Wishing You Luck."
Joe: "I have a really important soccer game today."
Bob: "Have Fun, IWYL"