From Wiktionary, the free dictionary
не- (ne-) + в- (v-) + терпе́ть (terpétʹ) + -ёж (-jóž)
невтерпёж • (nevterpjóž)
- being impatient (in anticipation of something), hard to bear, hard to be patient, hard to wait
- ему́ ста́ло невтерпёж ― jemú stálo nevterpjóž ― he couldn't bear it / wait any longer