Source : Webster's Revised Unabridged Dictionary (1913)

Headlong \Head"long`\ (?; 115), adv. [OE. hedling, hevedlynge;
   prob. confused with E. long, a. & adv.]
   1. With the head foremost; as, to fall headlong. --Acts i.
      18.

   2. Rashly; precipitately; without deliberation.

   3. Hastily; without delay or respite.

Headlong \Head"long\, a.
   1. Rash; precipitate; as, headlong folly.

   2. Steep; precipitous. [Poetic]

            Like a tower upon a headlong rock.    --Byron.

Source : WordNet®

headlong
     adj 1: excessively quick; "made a hasty exit"; "a headlong rush to
            sell" [syn: {hasty}]
     2: with the head foremost; "a headfirst plunge down the
        stairs"; "a headlong dive into the pool" [syn: {headfirst}]
     adv 1: with the head foremost; "the runner slid headlong into third
            base" [syn: {headfirst}]
     2: at breakneck speed; "burst headlong through the gate" [syn:
        {precipitately}]
     3: in a hasty and foolhardy manner; "he fell headlong in love
        with his cousin" [syn: {rashly}]