This is an uncommon English expression – it is an unexpected and surprising way to say “late 30’s” – that is, being between 36 and 39 years old. If we think that it’s a bad thing to get older, then as you get older, your status declines lower and lower. So, from age 30 to age 39, you get deeper and deeper.
Articles about This Is Us – S01E01 – Pilot
This English expression is a bit of a joke. If you are “wearing your birthday suit”, then you are naked. Your birth day suit is the suit that you were wearing when you were born. Of course, when you were born, you weren’t wearing a suit at all, because you were a naked baby.
The day that you were born was a very auspicious day, don’t you agree? Since your birthday was a very special day of celebration, people should dress formally for the occasion. That means everyone should wear a suit. However, babies are born naked. So the only suit that you can wear on your birth day is “nothing”.
In Season 1, Episode 1 of This Is Us, Rebecca is singing a birthday song to Jack and calls herself Shamu. REBECCA: [Singing] Happy 36th birthday to you, Happy 36th birthday to you, I have triplets inside of me, I am Shamu. Shamu Was a Famous Whale Shamu (pronounce it like you would say …