She folded up her letter, licking the envelope flap with relish.

她把信折起来，享受地舔着信封盖。

《柯林斯英汉双解大词典》

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

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