Rare literary word for one's inner thoughts or feelings. Modern Japanese would more commonly use 胸中, 心中, or 気持ち.
襟懐 is a word seen in old writing to express a person's inner heart.
The exact derivation is uncertain; the kanji 襟 (collar, bosom) and 懐 (bosom, heart) combine to suggest the inner region of the heart or mind.