Humble first-person pronoun used primarily by men in letters and formal writing. Conveys self-deprecation and respect toward the recipient.
I recently had the honor of reading your esteemed work.
愚生のつたない意見で恐縮ですが、一言申し上げます。
I apologize for my humble opinion, but allow me to say a word.
Compound of 愚 (foolish, humble prefix) and 生 (life, person). The exact historical derivation is uncertain, but it follows a common pattern of humble self-reference in classical Chinese-influenced Japanese.