Meaning of -FIRST in English

~ combines with nouns like ‘head’ and ‘feet’ to indicate that someone moves with the part that is mentioned pointing in the direction in which they are moving.

He overbalanced and fell head first.

COMB in ADV: ADV after v

Collins COBUILD.      Толковый словарь английского языка для изучающих язык Коллинз COBUILD (международная база данных языков Бирмингемского университета) .