The flap of the envelope had come unstuck.

信封的封口没粘牢张开了。

《牛津词典》

Please put the completed Form 26, together with any further reports required, into this envelope. Please seal it and place your signature or rubBer stamp over the junction of flap and envelope.

请把填好的26号表格随同其他所有进一步的诊断报告放进此信封内，然后封口封好，并在封口骑缝处签名或盖章。

youdao

His hand went into his pocket and a forefinger felt its way under the flap of the envelope, ripping it open in jerks.

他把手伸进兜里，一只食指摸索到信封的口盖，分几截把信扯开了。

youdao

Just as he starts removing the diagrams from the envelope in the back flap, he changes his mind and stuffs them back into the envelope.

BBC: The sex manual for ultra-Orthodox Jews

The moment was signature Steve Jobs: Standing on stage at the Macworld conference in San Francisco, Apple's chief executive pulled out an interoffice envelope and opened the flap.

FORBES: Magazine Article