grammar
Structure: adjective + 死了 is a colloquial intensifier meaning 'to death / extremely'. Also 饿死了 (starving), 热死了 (boiling), 笑死了 (hilarious). Hyperbolic, not literal.
register
Very casual. Use with friends, family, colleagues you're close to. In polite or formal contexts say 我很累 or 太辛苦了.
