It is Abigail Williams who is described in these terms. To dissemble means to mislead others by hiding your true feelings or motives. As the play progresses and Abigail's characterization deepens, the audience comes to recognize the accuracy of the statement that she has "an endless capacity for dissembling."
Abigail's deceptiveness is apparent in Act I when she initially denies practicing witchcraft with Tituba and the other girls. She coaches the girls to tell her uncle that the girls were only dancing and that Tituba had "conjured Ruth Putnam's dead sisters" when the truth is that Abigail had asked Tituba for a spell to kill Elizabeth Proctor.
Abigail even dissembles when she is with the object of her desire, John Proctor. She tells him in a private conversation in Act I that "we were dancin' in the woods last night." Abigail hides her true feelings and motivations: she is murderously jealous of his wife and intends to dispose of her in any way she can.
Abigail's lies and deception continue throughout the play until her exit is revealed in Act IV. She consistently casts herself as the innocent victim of others--Elizabeth Proctor in particular--instead of the ringleader in a deadly game.
The "endless capacity for dissembling" is a phrase used to describe Abigail. It is a reflection of her characterization. Abigail's characteristics show her to be manipulative in terms of her relationships with other people. Abigail's "endless capacity for dissembling" is one that allows her to conceal her true emotions for other motives. She displays an ability to pretend with other people in order to achieve what she wants. Her "endless capacity" for manipulation and, essentially, deceit is what drives the drama. Her ability to convince her uncle to advocate the presence of witches in Salem and Abigail's ability to convince the other girls to succumb to the power of accusation are part of her plan to control and gain John Proctor for herself. While others would think that there is a limit to such deceit, Miller's description of Abigail is one in which there is no end to her capacity to control others. Abigail's limitless vortex of cruelty and manipulation is part of the reason why Proctor's humanity is so vital, as it constitutes a response to the "endless capacity" for evil that exists in the world.