Ostensibly, "American Buffalo" is all about three bush-league criminals, who are plotting a petty heist that never comes off. But midway through the first act, one of them, Teach, played with nervous brilliance by Al Pacino, lets slip the real matter of David Mamet's prickly 1977 drama.

Nodding his head to the insistent beat of some internal drummer and jabbing his fingers at the floor for emphasis, he asserts, "One thing makes all the difference in the world--knowing what in the ---- you're talking about."

The line gets a big laugh in the Terrace Theater, where Mamet's play has settled in for a run through Sept. 25. Up to that moment--and indeed for the rest of the evening--Mamet's characters seem unintentionally bent on demonstrating their complete and abject inarticulateness. When they are not being inarticulate, they are being evasive. Sometimes they are both.

The apparent action in the play is slight. The proprietor of a ratty junk shop one flight down, Donny Dubrow (J.J. Johnston) recently sold a rare American buffalo nickel to a customer. Now he's thinking that there might be even more profit in breaking into the customer's home and ripping off the man's coin collection. To that end, he's had Bobby (James Hayden), a sorry young junkie who helps around the shop, put out a stake on the man's house, and he's drafted Teach, a fourth-rate crook, to handle some of the logistics.

No one quite trusts the other, however, and since they are all essentially bumblers, it is clear early on that the robbery will never occur. Bit by bit, as their self-styled facades unravel, the real concern of Mamet's play comes sharply into focus. All three are prisoners of language, men wrestling with syntax and vocabulary and going down for the count time and again. They see themselves as creatures of experience, logic and common sense--"businessmen" onto a "good business proposition." They know what they want to say and they have the swagger and force of their convictions. But they keep running up against words and the words betray them.

Picking up the phone and sounding out a prospective buyer for the hot coin collection that is about to fall into his hands, Donny asks with the gravity of a stockbroker, "If I can get ahold of some of that stuff you were interested in, would you be interested in it?"

"I'm calm," says the riled Teach at one point. "I'm just upset." And later, drawing the moral from a revelation of card-table cheating, he concludes, knowingly, "The whatchamacallit is always the last to know."

In the superlative production at the Terrace Theater, the spectacle of three hoods caught in a linguistic mine field can be keenly funny. At the same time, it is frightening. Frightening because the violence, the frustration and the anger locked within them cannot find a way out. Time and again they preface their thoughts with "All I'm saying is . . ." because what they are saying is coming out obliquely or backward.

The more the pressure builds, and words fail them yet again, the closer they edge to violent deeds. In the end, "American Buffalo" explodes with an act of bloody cruelty that seems totally gratuitous on the surface, but is, in fact, the only way Teach can break through the verbal straitjacket that hugs him as snugly as his slick gray shoes.

Mamet's command of lower-class speech with all its expletives undeleted is unfailingly accurate--so accurate as to suggest that he has tape recorders where the rest of us have ears. Yet the attentive theatergoer will find the dialogue ripe with unsettling resonance, especially as it is delivered by the first-rate cast. Spontaneity of this caliber is clearly the result of long and arduous rehearsal.

Unlike the recent parade of celebrity actors at the Kennedy Center, Pacino the star disappears in short order and you find yourself increasingly drawn in by the characteristics he has imagined for Teach--the tension that informs his step; the way he nervously pats his groin, as if his masculinity needed periodic reassuring; the ever-present concern for his hair, lest a flash of anger dislodge his swept-back coiffure. Pacino's timing is flawless, converting what could be the flat responses of a two-bit dolt into revelations of the disinherited. And his fury is awesome.

In their fashion, the supporting players match him at every step. Johnston projects a residual rectitude as the junk shop owner, and his compassion for the wan junkie may be the one glimmer of salvation in this dusty hell. And Hayden, his body a pathetic question mark, is strikingly effective as that sorry wreck, who holds to his lies because he's too dumb to wriggle out of them.

Arvin Brown's direction never lets you think for a moment that matters are anything less than real, and Marjorie Bradley Kellogg's set, cluttered with the detritus of civilization, could have been lifted from any urban ghetto. It is not mere happenstance that the clutter gets periodically in the characters' path. It is symptomatic of their plight. More than in a maze of old tires and broken-down furniture, the three sad souls of "American Buffalo" are trapped in the refuse and castoffs of language itself. AMERICAN BUFFALO. By David Mamet. Directed by Arvin Brown. Set, Marjorie Bradley Kellog; costumes, Bill Walker; lighting, Ronald Wallace. With Al Pacino, J. J. Johntson, James Hayden. At the Terrace Theater through Sept. 25.