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1. to be a sport
2. to go along and do what you are supposed to
From Reservoir Dogs :

(talking about tipping and the fact that tips are taxed)

Mr Pink: (...) Look, if you show me a piece of paper that says the government shouldn't do that, I'll sign it, put it to a vote, I'll vote for it, but what I won't do is play ball. (...)

Meaning he will not do like everyone and start tipping waitresses.
play ball by BelgianTake June 21, 2012
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"I'm gonna play ball with Robert tonight."
"Let's get drunk and play ball."
Play Ball by mlballstar April 24, 2011
In a relationship in which one partner is polyamorous, the act of the monogamous partner allowing the poly partner to occasionally have sexual relations with other partners. Similar to a “hall pass,” it shows respect, trust, and mutual understanding between the partners. Those who refuse to play ball are frankly immature, possessive, controlling, and often abusive to their partners.
I thought I’d never get to hook up with Bella, but luckily her boyfriend decided to play ball. He’s such a cool and understanding guy.
Playing Ball is like shooting with some one.

Play ball on their own 

When something is top of their game and no-one else comes even close to competing. Παίζουν μάππαν μόνοι τους. First used in the 2020’s by A.Dose
Oh my god. Have you seen the new couture show? Amazing! They play ball on their own!

If there's grass on the field, play ball

A term basically saying that it's alright to at least be attracted to a person as long as they've gone past puberty. Obviously "grass" refers to their "bush" or pubic hair. Would typically be applied to, for instance, a girl maybe 13 years old and up. Can often be used in reference to actual sex (intercourse) but not necessarily.
Man#1: What's the youngest chick you would ever be into at your age? I mean what do you think is "okay" to fantasize about?

Man#2: Hey man. Like my coach always said: if there's grass on the field, it's time to play ball.

if there is grass on the field play ball 

if a girl is old enough to have pubic hair (shaved or unshaved) they are old enough to have sex.
Bob: Do you think Mary is old enough to shag?
John: If there's grass on the field play ball