A person's opinion that is not valid in the writer's eyes.
This can also be used on people (Possibly to say they r no lifes)
Example 1: So.. You say Sugar x Ella is a valid Total Drama ship? That is so mid!
Example 2: ngl Jellybean is mid. - Meowbahh, not knowing she is also mid.
Mid is usually a term to describe something unsatisfactory. Weather that be the look of a bird on a night out, or the quality of chips at the local.
"Oh god the reply was mid, cheers Taya"
Something average, nothing special
Waiter: How did you like the food?
Gordon Ramsay: Eh, it was pretty mid.