内心|ないしんnaishin[0]heiban (flat)

1. no-adjectiveinner thoughts; real intention; inmost heart; one's mind
   政治家せいじかは決けっして内心ないしんを見みせない。↗

   Politicians never tell us their inner thoughts.
2. noun, adverbat heart; deep down; on the inside
3. noun, mathinner center (centre)

• Synonym胸襟きょうきんWrittenWritten word for the heart or bosom, mostly in 「胸襟を開く」, to open up to someone; not a general substitute for 「内心」.
• Synonym心しんNeutralThe broadest word for the mind and heart; 「内心」 narrows this to thoughts kept hidden deep down and not shown to others.
• Synonym心根こころねWrittenOne's inner nature or disposition, as in 「優しい心根」; stresses character, while 「内心」 stresses thoughts hidden at the moment.
• Related胸むねNeutralMostly the physical chest, but also the heart's feelings, as in 「胸が痛む」; carries a bodily, felt nuance that 「内心」 lacks.
• Related心頭しんとうWrittenWritten word for the heart or mind; best known from the set phrase 「怒り心頭に発する」. Not a casual substitute for 「内心」.
• Related心こころNeutralThe broadest word for the mind and heart; 「内心」 narrows this to thoughts kept hidden deep down and not shown to others.
• RelatedハートNeutralLoanword for the heart and feelings, also a heart shape or the hearts suit; light in tone, unlike 「内心」's hidden true thoughts.
• Related胸三寸むねさんずんWrittenWritten word for the plans or calculations one holds in mind, as in 「胸三寸に納める」; not interchangeable with 「内心」.
• Related胸懐きょうかいWrittenWritten and rare; thoughts and feelings held in the breast. Far too literary to stand in for 「内心」 in everyday speech.
• Related胸臆きょうおくWrittenWritten and rare; one's innermost thoughts and feelings. A literary word, not an everyday substitute for 「内心」.
• Related御心みこころFormalHonorific for another person's will or God's will, as in 「神の御心」; formal or religious, unlike the neutral 「内心」.
• Related心髄しんずいWrittenWritten and rare; the essence or core of a matter, or the bottom of one's heart. Literary, not a casual stand-in for 「内心」.
• Antonym表面ひょうめんNeutralThe surface or outward appearance of things or people; the direct opposite of 「内心」 — what shows, not what is hidden inside.
• Antonym外面がいめんWrittenWritten word for outward appearance and the manner one shows others; the opposite of 「内心」, one's hidden real thoughts.

Use 「内心」 for true thoughts kept hidden deep down, as in 「内心ほっとした」. For the mind or heart in general pick 「心」; for feelings felt in the body, 「胸」. 「ハート」 is a casual loanword for the heart or a heart shape. 「心頭」「胸三寸」「胸襟」 appear in set phrases, 「御心」 is formal for God's will, 「心根」 points to character, and 「胸懐」「胸臆」「心髄」 are rare literary words. 「表面」 and 「外面」 are the opposites.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 心こころの中なか。
2. 多角形たかっけいの全すべての辺へんに接せっする円えんの中心ちゅうしん。