head ‧ long /ˈhedlɒŋ $ -lɒːŋ/ BrE AmE adverb
[ Date: 1300-1400 ; Origin: headling 'head-first' (14-17 centuries) , from head + -ling 'in a particular direction' ]
1 . rush/plunge headlong into something if you rush headlong into something, you start doing it too quickly without thinking carefully
2 . with your head first and the rest of your body following SYN headfirst :
I fell headlong into a pool of icy water.
3 . very quickly, without looking where you are going:
Mortimer almost ran headlong into a patrol.
—headlong adjective