忝 is a classical humble word: to disgrace, or to be unworthily included. It survives in set self-deprecating phrases like 忝列 and the classical 无忝 (to bring no shame to one's forebears). It joins a heart element with 天 for the sound.
register
Literary and humble — used in formal modest expressions, not in everyday speech.
Components
radical
⺗xīn
heart; mind
Bottom-heart form ⺗, the squashed form of 心 used under a character. Shame is felt in the heart, so 忝 carries the heart radical, like 恭 and 慕.
