Plain English Meaning

The wing pointing toward where the wind is coming from. If the wind is blowing from your left, your left wing is the upwind wing.

Full Definition

The wing on the side of the airplane facing into the wind during a crosswind takeoff or landing. In a crosswind, this is the wing the wind is striking first, and it tends to be lifted by the relative wind unless the pilot deliberately holds it down with aileron input.

Context Anchor

Used during taxi, takeoff roll, landing roll, and other ground operations when wind is coming from the side.

Derivation

Upwind means 'toward the of the wind' — the direction the wind is coming from. The upwind wing is simply the wing on that side of the airplane.