Acronym for Mount Taint Bike, which is old slang for a Mountain Bike. Riding a bicycle on rough terrain causes repeated trauma between the rider's legs; in male cycling enthusiasts, this often leads to irritation and swelling of the prostate
. This would result in a characteristically swollen perineum
(a "mount taint
"), the external manifestation of both local trauma and the enlarged prostate beneath.
With the increased popularity of padded bicycling shorts, the "mount taint" phenomenon has decreased in frequency (though by no means disappeared), and many cyclists nowadays are unaware of the origin of the term MTB, believing instead in a folk etymology that describes it as a nonsensical acronym of *"mountain bike" ("MounTain Bike"?).
Albert: Hey Bob, what's up? You don't look so good!
Bob: No kidding! I had to see my proctologist
last week... now I get why they are called MTBs!