If you want to play hardball, you’re not messing around. You’re a shrewd negotiator and you’ll do anything to get what you want. Or maybe you just want to play baseball, which is also known as hardball.
You can use the slightly old-fashioned hardball for this all-American sport, particularly to distinguish it from softball, which is played with similar rules but a larger, slightly softer ball. You can also use the word informally to mean "ruthless methods." So if a businessperson, politician, or journalist plays hardball, they’re being super competitive and try to win at all costs.