What the heart thinketh, the tongue speaketh.

A person's speech may disclose what occupies their inner thoughts or feelings. Even when someone tries to conceal an attitude, their words can reveal underlying wishes, judgments, or concerns.

The saying treats the heart as the seat of inward thought and feeling, and the tongue as its outward expression. It cautions listeners to notice what speech can unintentionally betray, without treating this as a certain way to judge private thoughts.

Use it when remarks, especially spontaneous or repeated ones, seem to reveal a person's genuine feelings or preoccupations. Its archaic verb forms give it a traditional tone. It can be observational or mildly admonitory, but should not imply that every spoken word perfectly states someone's inner life.