心底 しんそこ
shinsoko

Noun

1. bottom of one's heart; depths of one's mind; innermost thoughts; real intentions

かれ心底 しんそこ善人 ぜんにんだ。
karewa shinsokowa zenninda.
He is a good man at heart.
Adverb

2. from the bottom of one's heart; wholeheartedly; truly; sincerely

Applies to 真底
きみには心底 しんそこ失望 しつぼうしたよ。
kiminiwa shinsokoshitsubō shitayo.
I'm very disappointed in you.
Other forms: