Lauryn Hill -- Terrifying Threats Made Me Quit Music AND My Taxes

Lauryn Hill claims she stiffed Uncle Sam on a massive tax bill for the same reason she fell off the map after the '90s -- scary threats to her and her family.

Lauryn just filed legal docs asking for mercy before her sentencing later this month, claiming there was a good reason she failed to file tax returns for 2005, '06 and '07 ... when she earned a total of more than $1.8 million. She already pled guilty and faces several years in prison.

According to docs ... Lauryn stopped paying taxes "when she withdrew from society at large due to what she perceived as manipulation and very real threats to herself and her family."

She's not specific about the threats -- but it's a new explanation for her mysterious disappearance from the music scene after her rise to fame in the '90s.

Lauryn previously claimed she went underground to get back in touch with her musical roots.

Lauryn's asking the court to consider a bunch of other factors before her sentencing -- including her claim that she can pay off the debt much faster if she's not behind bars.

Lauryn is begging for probation. A judge will be merciful -- or not -- on April 22.