The MLB legend learned that the hard way when he and his significant other, María Yeribell Martínez, were throwing a party recently to reveal the gender of their unborn baby.

The two came up with a baseball-themed plan to make the announcement ... where María would throw a pitch to Ortiz that he was then supposed to blast into either pink or blue smoke.

But, check out footage from the bash ... Ortiz -- a 10-time All-Star with 541 career home runs -- completely whiffed on María's inside heater!!

To his credit, Ortiz and his partygoers all took the gaffe in stride -- they still celebrated with blue confetti while the former Red Sox designated hitter threw his arms into the air and jumped with joy.

Of course, social media has gone crazy over the whiff ... with many hilariously joking María had read the scouting report on Ortiz prior to throwing him the cheddar.