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

Headlong
In this humorous novel about the art world and its excesses, the appearance of a lost painting that may or may not be by Bruegel occasions a series of exercises in deceit, betrayal, and blind optimism when the man who comes across it at the home of a neighbor resolves that he must possess it at all costs.

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1. \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.
   

2. \Head"long\, a.
   1. Rash; precipitate; as, headlong folly.
   2. Steep; precipitous. [Poetic]
            Like a tower upon a headlong rock.    --Byron.