天高地厚 is a four-character idiom that literally means 'the sky is high and the earth is deep' — the vastness of the universe. It is most often used figuratively to describe the profound depth of someone's kindness or love (like 天高地厚之恩 'favor as vast as heaven and earth'). In the negative form 不知天高地厚, it criticizes someone as ignorant, arrogant, or unaware of their own limits — a common, useful expression. Use it in both positive (deep gratitude) and negative (audacity) contexts.