A person who has great trouble SMS messaging. These people typically think texting is either stupid or do not do it enough to be very good at it. An SMS response from a textually impaired person typically takes a long time to receive, even if the message is simply 'yes' or 'no'.
(Sent in a text message) 'You can call me if you're that textually impaired.'
"I only call my parents because they're textually impaired."