1. be harsh or relentless in order to achieve a certain result
he's a nice guy, but can play hardball when he needs to
'Play hardball' is an expression meaning to maintain a firm and determined position in order to get what you want. If you 'play hardball' you may behave in an unpleasant or threatening way. This expression is often used to talk about someone who is is is difficult to negotiate with because of being unwilling to exchange or relinquished anything. This expression is more popular in the US it is considered an informal phrase.