i.e. someone who isn't able to use a mobile-phone, or a DVD player might be called a twanky. If it's the device that's at fault, e.g. an old out-dated phone, then it would be the device/phone that would be twanky and not the person using it.
I don't know where this colloquialism originates - perhaps it's a reference to Widow Twanky.
The best way of describing it would be to say that you can use 'twanky' anywhere where you might say instead "my granny could do better".
S/he's a bit twanky.
It's very old, twanky technology.
( Heated over a flame the meth melts and vaporizes, The pipe is spun to keep the meth from burning while the vapors are inhaled)