With the head going first or in front of the rest of the body; = headlong, adv. 1.

Directly, squarely; precisely, with absolute accuracy. Cf. fair, adv. 7.

With the head or front directed towards something, esp. so as to collide or come into contact; (of two things, esp. vehicles) so as to face one…

From directly in front; head-on. Later also without reference to physical motion or direction: directly; so as to make a direct or significant impact.