Donald Trump's got a thing about Arnold Palmer's penis ... and he fixated on it for a few awkward, yet humorous, minutes during a rally in the golf legend's home town.
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.