adverb taking the 'to' particle
very deeply; profoundly
Describes a deep state, often used with verbs like 眠る (sleep), 考える (think), or 感謝する (thank). The adverbial form 深々と is common.
彼は深々と眠っている。
He is sleeping very deeply.
その言葉に深々と感謝した。
I felt profoundly grateful for those words.
深い is the basic adjective meaning 'deep'. 深々 is an adverbial form emphasizing a very deep state or manner.
Reduplication of 深 (ふか, 'deep') with the adverbial suffix 々, yielding an emphatic adverbial form.