adverb
from the beginning; without hesitation; flatly
Used for doing something right from the start, or for rejecting/refusing outright without any hesitation. Often with verbs like 断る, 否定する, or 決めつける.
彼の提案を頭から否定するのはよくない。
It's not good to reject his proposal out of hand.
その話は頭から信用していなかった。
I didn't believe that story from the very beginning.
adverb
Emphasizes that something is done or applies to the whole thing, without exception. Often used with verbs like 無視する, 忘れる, or 変える.
彼は私の忠告を頭から無視した。
He completely ignored my advice.
その計画は頭からやり直す必要がある。
The plan needs to be redone entirely from scratch.
adverb
head-on; head-first
Describes a direct, often physical collision or confrontation. Also used metaphorically for tackling a problem directly.
車がガードレールに頭から突っ込んだ。
The car crashed head-on into the guardrail.
彼は難しい問題に頭から取り組んだ。
He tackled the difficult problem head-on.