adverb
thoroughly; from beginning to end; completely
Emphasizes doing something consistently and without exception from start to finish. Often used in formal or emphatic contexts.
彼は徹頭徹尾、自分の意見を曲げなかった。
He stuck to his opinion from beginning to end.
この計画は徹頭徹尾、見直す必要がある。
This plan needs to be thoroughly re-examined.