headlong (adv.) Look up headlong at Dictionary.com
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.