means A score in American or Canadian football, worth six points, achieved by getting the ball into or across the opponent's end zone.

from A straightforward compound of "touch" and "down," born in 19th-century rugby football, where a player literally had to touch the ball down to the ground behind the goal line to score — hence the name. American football inherited the term, then quietly dropped the requirement that anyone touch anything down, leaving the word as a fossil of an older rule. The aviation sense — a plane's wheels meeting the runway — is a later, literal borrowing of the same image.