If you need a reminder -- Bobbitt's name became notorious in 1993 when his wife Lorena cut off his penis while he was asleep and threw it out her car window. He got it reattached successfully, but for more on that story ... look it up.

As for his current foot problem ... John tells TMZ he was working construction in Buffalo in 2013 when he stepped on a nail, which went through his boot and punctured a toe on his left foot. He says it became infected a couple months later.

Bobbitt says he got treatment at 2 different hospitals, but a doctor later diagnosed him with osteomyelitis -- an infection of the bone in his left foot.

We're told John has been aggressively treated with antibiotics, but the infection never fully healed. Now, he says he's back in a hospital in Las Vegas, and as you can see for yourself ... it doesn't look good!!

Bobbitt says doctors are telling him the bone is so infected they need to determine whether or not to amputate.

JWB's got his fingers crossed for the left foot decision, especially because -- get this -- he already lost a toe on his right foot last year! He says that little piggy was also infected after an accidental cut.

We don't believe in curses, but ... damn.