A$AP Rocky has at least one secret he does NOT want coming out in his assault trial ... so much so, he tried to stop his pal A$AP Twelvyy from answering a question on the stand!

Prosecutors grilled Twelvyy during Friday's court proceedings, showing him a photo of Rocky's bed with the letters "AWGE" emblazoned on the furniture, and asked what the letters mean -- but before Twelvyy could respond, Rocky yelled out, "Don't say!"

The outburst created some chaos ... Twelvyy would only say, "It means AWGE," which pissed off the prosecutor, who asked the judge to force the witness to be more forthcoming. When Twelvyy kept stonewalling, Rocky's attorney, Joe Tacopina, asked for a sidebar with the judge.

Aaaand, just as that was going down, Rihanna was strolling into the courtroom -- talk about dramatic entrances! She was mum outside the courthouse, but once inside she met Rocky in the hallway, gave him a hug ... and they went back inside as court resumed.

RiRi, rocking a gray suit jacket and pants with a yellow scarf, watched as prosecutors pressed Twelvyy even more -- but things got even more heated.

Ultimately, Rocky got his way ... Twelvyy never rolled over on the meaning of the 4 letters. For the record, AWGE is a creative agency Rocky formed in 2014, but we really can't tell ya more than that. The agency's own website says, "Rules: #1 Never reveal what AWGE means."