Speaking on SiriusXM's "Conan O'Brien Needs a Friend," Adam and host Conan discussed how 21st-century celebs didn't quite leave them fangirling like the ones from back in the day did -- but according to the actor ... one modern star broke that rule -- Tay Tay!!

Adam explained that he still gets "a little f***ing jumpy" around the singer cause of what she means to his teen daughters Sadie and Sunny ... and he doesn't want to risk blowing their chance to have an awesome experience by acting all awkward around TS.

In fact, it's like his comedy movie characters come to life when he's trying to get Taylor's attention ... 'cause he says he's super loud and excited!!

Conan totally got where Adam was coming from here, BTW ... adding TS was on a whole other level now with her superstardom reaching crazy heights and her Travis Kelce romance hitting headlines.

Adam went on to compare Swift's success to "Beatlemania" in the '60s ... adding she has so many smash hits, and there's not a word his kids don't know.

He's had plenty of opportunities to sing along with them, too ... last year, he took his daughters to Taylor's "Eras" tour in L.A., and they also hit up the "The Eras Tour: The Movie" premiere. So yeah, AS is a well-anointed Swifite via proxy, thanks to his kids.