When someone sends a meaningless, typo filled text usually because they were in a rush and their fingers are too stubby or fat to easily type on their phone. Analogous to the idea of a T-Rex attempting to text with its tiny hands.
Bob: Dude, did you see this gibberish Nesta sent to our whatsapp group?
Marley: Hahah, yeah man, classic t-rext! He should really lose some weight.