1To be disappointed; to feel let down or discouraged (often used as a noun or in the form がっかりする).
試験に落ちてがっかりした。
I was disappointed to fail the exam.
約束の時間に来なかったので、友達はがっかりしていた。
My friend was disappointed because the person didn't show up at the promised time.
期待していた映画がつまらなくてがっかりした。
I was disappointed because the movie I expected was boring.
採用されず、彼は大きながっかりを感じた。
He felt great disappointment at not being hired.
2(Dated) To become weakened or drained of energy, as from illness or exhaustion. This sense is essentially unused in contemporary Japanese — today がっかり almost always means 'disappointed' (sense 1).