Dining at the Y is a euphemism for performing cunnilingus. "Dining" refers specifically to performing oral sex on a person with a vagina, an act often referred to as "eating out." The second part of the phrase, "at the Y," alludes to the shape of the receiver's body. The person receiving oral sex looks like a Y, with their legs open and their torso forming the letter’s base.
The phrase Dining at the Y can get used for comedic effect when talking about sex. It can also be useful for discussing sex in a situation where describing sex acts in graphic detail isn’t appropriate. For example, someone might prefer using this term when they’re talking to people who want to hear such things at a restaurant or the office.
Dining at the Y is often abbreviated to the initialism DATY. This abbreviation is commonly seen in online forums, internet chats, and text messages. For example, you might text someone you’re dating, "See you later, can't wait to DATY! ;-)" Sex workers who allow or perform cunnilingus may also use the acronym DATY on their menus. Some sex workers who don’t offer full-service sex offer DATY as it is a lower risk activity when it comes to specific STIs than penetrative sex. Dining at the YMCA is a long-form version of Dining at the Y.
You shouldn't confuse DATY with DATO, a similar initialism that stands for Dating at the Orifice. While DATY is shorthand for oral sex, DATO is short for analingus.
Lunching at the Lazy Y is a similar term to Dining at the Y. Like Dining at the Y, this phrase also references “eating out” and the Y shape of the body enjoying cunnilingus. While Lunching at the Lazy Y is a less common phrase, the terms can get used interchangeably to refer to cunnilingus. Having breakfast in bed is another food-related synonym for Dining at the Y.