David Ortiz Blasts Mike Fiers For Whistleblowing On Astros, 'Snitch'

David Ortiz just went OFF on Mike Fiers for blowing the top off the Astros' cheating scandal ... with Big Papi straight-up calling the ex-Houston pitcher a "snitch."

"I'm mad at this guy, the pitcher that came out talking about it, and let me tell you why," Oritz said at Red Sox camp Thursday. "Oh, after you make your money, after you get your ring, you decide to talk about it?"

"Why didn't you talk about it during the season when it was going on? Why didn't you say, 'I don't want to be no part of [this]?' So, now, you look like a snitch."

As we previously reported ... Fiers blew the whistle on his former team just a few months ago in an interview with The Athletic -- revealing the 'Stros cheated their way through the 2017 season.

Of course, the scandal didn't help Fiers personally pitch better -- the scheme only helped hitters at the dish -- but Big Papi is still railroading the guy for not speaking up while it was all going down.

"That's my problem," Papi said. "Why nobody say anything while it was going on?"

For his part, Fiers has seemed to understand the criticism in all of his interviews this month ... saying to the San Francisco Chronicle just this week, "I said from the beginning, 'I'm not away from this.'"