寡人 literally means 'the person of little virtue' or 'the lonely one.' Pre-Qin kings and lords used it as a humble self-reference, emphasizing the isolation of ruling. After the Qin dynasty, 朕 (zhèn) became the standard imperial pronoun.
register
Strictly classical / literary. In modern usage it appears in historical dramas or humorous contexts where someone pretends to speak like an ancient king.
