The story
All such constructions, along with elbow-deep, knee-deep, waist-deep, neckdeep, and so on, rely on the same metaphor: the depth to which a wader or swimmer is submerged in a body of water.
Etymology from Wiktionary, CC BY-SA 4.0.
up to here
Today it means: Used other than figuratively or idiomatically: see up to, here. Full definition →