A short form of 'specify', which means to say exactly what is needed or wanted.
When you spec something, you are being very clear about what you want.
Examples:
Can you spec the color you want?
I need to spec the size of the shirt.
She specs the type of cake she wants for her birthday.
I spec the kind of pizza I want, and it always has a lot of cheese!
He specs the car he wants to be very fast and very red.
We spec the movie we want to see, and it's always a funny one.