adj. A symbolic description used to indicate when one thing is extremely close to another or when an event is imminent.
2002
Next flight's not for a couple of hours. Go home. Half undress. Go to bed. Oversleep. Jump up. Race to car. Snow's still falling. Ice is thicker, slicker. From the top of the hill next to my apartment, I watch with dismay as a station wagon just yards ahead loses control, ending up thisclose to a tree.
1982 (earliest)
[T]he Bucs' Williams (12) is thisclose to putting everything together."
This term is based on the idiomatic gesture of holding one's thumb and forefinger very close together while saying emphatically that one thing was "this close" to another.