1810, member of a gang of murderers and robbers in India who strangled their victims, from Marathi thag, thak cheat, swindler, Hindi thag, perhaps from Sanskrit sthaga-s cunning, fraudulent, possibly from sthagayati (he) covers, conceals, from PIE root *(s)teg- cover (see stegosaurus).
It may also occur in one ear or both ears. The most common reason for muffled hearing is conductive hearing loss that can be caused by a variety of factors or medical conditions including a build up of ear wax and ear infections. In most cases, however, it is treatable and does not lead to permanent hearing damage.
Mortartube's absolutely right - technically known as in-ear monitors. sparkyharky. The ear pieces are actually there to provide feedback of the artists own voice. Have you never listened to someone singing along to a song whilst wearing headphones with the volume turned up (if you havent, try it ... its fun!) when you can't hear your own voice you very quickly start singing flat.