head first

adverb

1 she dived head first into the water:

head foremost, headlong, on one's head.

▷antonyms feet first.

2 don't plunge head first into a new relationship:

without thinking, without forethought, precipitately, precipitously, impetuously, rashly, recklessly, carelessly, heedlessly, hastily, in haste, head over heels, headlong.

▷antonyms cautiously.

head first Oxford Dictionary of English