Big Papi says he's NOT gonna unretire and hit for the Boston Red Sox this season -- despite a big endorsement from his legendary ex-teammate, Pedro Martinez

Pedro got everyone talking Monday when he tweeted out, "Big Papi is working out, and I know he is in baseball shape, and the @RedSox could use a bit of his bat."

So, when we saw David Ortiz at CC & Amber Sabathia's PitCCh At Foundation charity event in NYC, we had to ask if a comeback was on the table. 

Unfortunately, Ortiz shoots that down ... telling TMZ Sports he's too busy enjoying his retirement and watching quality television programming.