I’ve often heard that saying from conservatives. The implication is that being open minded makes you gullible or susceptible to believing just anything eg, evolution, gender rights, gay marriage, etc.
to me, openmindedness means being able to entertain an idea without accepting it. So I am open minded towards woo in the sense that I will entertain the idea that it might work, and I'll propose empirical tests. But I don't accept it prima facie. But just accepting stuff is not being openminded, that's being gullible. Gullible != openminded.