Mellow out Definition 1 of 3Pro Tip 1/3Stress Reduction Use 'mellow out' when someone needs to reduce stress or anxiety. Take a deep breath and mellow out before the interview. Pro Tip 2/3Leisure Time Mention 'mellow out' when talking about relaxing during leisure or downtime. This weekend I'll mellow out at the beach. Pro Tip 3/3Shared Relaxing 'Mellow out' can be used when suggesting a group of friends calm down together. Guys, let's just mellow out and watch a movie.