also: つくつく
adverb, adverb taking the 'to' particle
Expresses a strong, often negative feeling that one has come to realize or feel thoroughly, such as disgust, regret, or weariness. Often used with verbs like 思う, 感じる, 嫌になる.
自分の失敗をつくづく後悔した。
I deeply regretted my mistake.
彼の無責任さにはつくづく呆れる。
I am utterly disgusted by his irresponsibility.
adverb, adverb taking the 'to' particle
Describes observing or examining something with great attention and thoroughness. Common with verbs like 見る, 観察する, 考える.
彼はその絵をつくづくと眺めた。
He gazed intently at the painting.
つくづく考えた末、会社を辞めることにした。
After thinking it over carefully, I decided to quit the company.