head firstEnglishOxford American Thesaurushead 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.