Tyrone Briscoe, the 18-year-old southwest Washington man who once boasted that robbing a bank "ain't no big thing," pleaded guilty in D.C. Superior Court yesterday to the armed robbery of a savings and loan association office last fall.

In a hearing before Judge Sylvia Bacon, Briscoe described how he and an accomplice robbed the Northwestern Federal Savings and Loan office at 1617 K St. NW on Oct. 17.

Briscoe, who was arrested for bank robbery five times in the last six months of 1978, appeared in court as part of a plea bargain in which he agreed to plead guilty to both armed robbery and bank robbery if the government would drop another bank robbery charge and a charge of carrying an unlicensed pistol.

Assistant U.S. Attorney E. Thomas Roberts said Briscoe is scheduled to plead guilty to bank robbery in U.S. District Court on Tuesday.

Ladd Leavens, Briscoe's court-appointed attorney, said Briscoe's decision not to fight the charges against him was a practical matter.

"We felt that with all the charges pending, he'd inevitably be convicted of something," Leavens said. "The government had a strong case. And Briscoe, who is very realistic, felt he would get less time by entering a plea than if he were convicted in a trial."

Briscoe is scheduled to be sentenced April 17.