late 14c., headling, also headlings, "headfirst (downward); headlong (forward); without thinking, hastily," from hed "head" (see head (n.)) + adverbial suffix -ling. Altered by c. 1400 to conform with sidelong, etc. Its true companions are now mostly obsolete: darkling, backling, flatling, etc.
1. She missed her footing and fell headlong down the stairs.
2. The country, they say, will inevitably now plunge headlong into decadence.
3. The army was in headlong flight.
4. She fell headlong into the icy pool.
5. The little boy fell headlong down the flight of stairs.