The line is spoken by Mrs. Fainall in act 2, scene 4. She is responding to Mirabell's less than gallant—but painfully accurate—remarks concerning Lady Wishfort, Mrs. Fainall's mother. Mrs. Fainall has already remarked that her mother will do just about anything to gain a husband, making her a perfect pawn in Mirabell's plan to blackmail Lady Wishfort into allowing him to marry Millamant. That is when Mirabell makes the following snide remark about Lady Wishfort, which emphasizes her desperation to get hitched:

Yes, I think the good lady would marry anything that resembled a man, though 'twere no more than what a butler could pinch out of a napkin.

It is then that Mrs. Fainall utters the line about female frailty. What she means is that when a woman gets old and desperate, she becomes emotionally weak and vulnerable. This feeds an appetite for false love now that the chance for true love has long since gone.

This quote is uttered to Mirabell by Mrs. Fainall in this masterpiece by Congreve that focuses on the double standards and hypocrisy of the upper classes. There is of course massive humour and irony in having Mrs. Fainall uttering these words, as she is hardly able to tut disapprovingly at the weakness of her own sex when she is currently engaging in an adulterous relationship that clearly reveals the same "female frailty" that she bemoans so loudly at this particular juncture in the play.

In fact, if we examine this quote more deeply, we can see that in a sense it acts as a kind of motto or thematic quote for the entire play, as the play itself concerns the way in which truth is decaying and replaced by falsity in the lives of so many characters in terms of what they say and do and who they pretend to be. Truth is in very short supply in this play, and the description of truth "decaying" is therefore very pertinent and relevant.