The GOP presidential candidate did his ad-lib thing Saturday night in Latrobe, PA ... and, although his campaign suggested he'd start previewing his closing argument for why voters should send him back to the White House -- he instead opened with big d**k jokes.

Trump reportedly spent about 12 minutes saluting Latrobe's hometown hero, notably praising Palmer's manhood. The former prez said, "Arnold Palmer was all man, and I say that in all due respect to women. And, I love women, but this guy ... this is a guy that was all man.”

After the misogyny, it got even weirder as Trump talked about AP's genitals impressing other golfers in the shower -- "They came out of there, they said, ‘Oh my God. That’s unbelievable.' I had to say. We have women that are highly sophisticated here, but they used to look at Arnold as a man.”

Trump mused on how much more fun his rally would have been if Arnold -- who died in 2016 -- could have been there.

He eventually did talk about politics, and take shots at VP Kamala Harris -- standard political rally stuff -- but the takeaway was all about the Palmer family jewels.