Jermaine Jackson -- That Hair Is Tight ... Soooo Tight

Jermaine Jackson's hair is keeping a secret ... and it has to do with The Jackson Five.

We ran into Jermaine in Calabasas Thursday ... and what he told us is mind-bending. Look at his hair today, and look at a picture of him back in the day when he sang with Michael.

Is it really possible by un-greasing Jermaine he could go back to his ABC days?

Mind-bending, right?