Adj + 得不得了 (extremely)

The final whistle

• Your school team has just won the city championship after a penalty shoot-out.
• team member队员duì yuán(plural)们mento move emotionally激动jī dòng(degree marker)得deterrible不得了bù dé liǎo。 — "The players were beside themselves with excitement."

Adj / feeling verb + (degree marker)得deterrible不得了bù dé liǎo

• Adj + (degree marker)得deterrible不得了bù dé liǎo = "extremely, terribly": 热得rè déterrible不得了bù dé liǎo, busy忙máng(degree marker)得deterrible不得了bù dé liǎo, happy高兴gāo xìng(degree marker)得deterrible不得了bù dé liǎo.
• (degree marker)得de links the adjective to terrible不得了bù dé liǎo (literally "beyond what can be stood"), which ends the clause.

the subway地铁dì tiě(in)里licrowded挤jǐextremely得不得了de bù dé liǎo。

Good and bad extremes

• Bad: today今天jīn tiān热得rè déterrible不得了bù dé liǎo，I / me我wǒerhua variant of 一点一点儿yì diǎn éralso也yěunexpectedly不想bù xiǎngto go out出门chū mén。 — "It's unbearably hot today; I don't want to go out at all."
• Good: to hear (sth said)听说tīng shuōyou你nǐwant / will / need要yàoto come来lái，child孩子hái zi(plural)们menhappy高兴gāo xìng(degree marker)得deterrible不得了bù dé liǎo。 — "When they heard you were coming, the children were overjoyed."

the children孩子们hái zi menhappy高兴gāo xìngextremely得不得了de bù dé liǎo。

One degree word, and (degree marker)得de is needed

terrible不得了bù dé liǎo on its own

• Alone, terrible不得了bù dé liǎo means "terrible, serious": terrible不得了bù dé liǎo(completed / change)了le，on fire着火zháo huǒ(completed / change)了le！ — "Oh no, there's a fire!"
• terrific了不得liǎo bù dé is different: "amazing, remarkable": he / him他tāreally真zhēnterrific了不得liǎo bù dé — "He's really something."

terrible不得了bù dé liǎo了le，on fire着火zháo huǒ了le！

Recap

Read the full reference: HSK 5 grammar — Adjectives.