na-adjective, noun which may take the genitive case particle 'no'
extremely grateful; deeply obliged
Very formal and somewhat stiff expression of deep gratitude, often used in business letters or formal speeches. Conveys a sense of being overwhelmed by the kindness or favor received.
ご厚情を賜り、恐縮至極に存じます。
I am extremely grateful for your kindness.
このたびは過分なお心遣いをいただき、恐縮至極でございます。
I am deeply obliged by your excessive consideration this time.
Compound of 恐縮 (kyōshuku, 'feeling obliged or apologetic') and 至極 (shigoku, 'extremely'). The combination intensifies the sense of being deeply indebted or grateful.