Matty Healy's throwing some low-key shade at Taylor Swift, seemingly calling her out for everything she writes about ... and, our "TMZ Live" team is here to read through the lines.

Healy gave an interview during which he said he has no interest in writing about relationships ... clearly referring to their short-lived romance last year after Taylor and Joe Alwyn split.

Plus, T-Swift's known for writing about all of her past romances ... including Matty Healy, who many believe is the subject of the song "The Smallest Man Who Ever Lived" -- no wonder he's throwing shade.

But, seems Matty got the last laugh here ... 'cause our guys say he used one word that's gotta cut like a knife. Watch the full clip to hear which one.