得不得了 after an adjective cranks it to the extreme — "incredibly …, … to death." On its own 不得了 means "[oh no,] terrible!", but as a complement it just intensifies, good or bad.
Structure: Adjective + 得不得了
today今天cold冷terribly得不得了how,怎么can会so这么cold冷?
It's terribly cold today — how can it be this cold?
the promotion升职made让him他happy高兴extremely得不得了。
Getting promoted made him over the moon.
Remember: 得不得了 = "…like crazy." It swings both ways — 高兴得不得了 (over the moon) and 累得不得了 (dead on my feet) — and hits harder than 很 or 非常.
Watch out
- It needs the 得: 高兴得不得了, not 高兴不得了.