This quote comes from Act I scene 1 of this play, and is said by Fainall to Mirabell, who has just beaten him at a game of cards. Fainall says that he will play Mirabell again when he is more focused on the game. He recognises that Mirabell is not truly focused on the gambling session they are having, and this is when he says this quote. For Fainall, he wants to enjoy the revenge he anticipates having over Mirabell, and if Mirabell is "cold" or not thinking about losing, this will lesson the joy of the revenge that Fainall will enjoy.
Of course, as with many lines in this excellent play, this line contains massive irony, as Fainall does not get to have his revenge. Mirabell is the person who continues to metaphorically "win" against Fainall in the conflict that they have during the play and to also enjoy the revenge that he gets on Fainall. Fainall never sees the revenge that he talks about in this quote at the beginning of the play. He is shown to be constantly outwitted by Mirabell.