Welche Bedeutung steckt hinter der hier vorgestellten Vokabel aus dem Bereich Emails?

The managing director, in an email to his staff: 'I was very happy to see so many of you at our anniversary event last week. It was such a big contrast to the erstwhile get-togethers we had when the company was first founded...'

erstwhile

Do you know what it means?

• happening only once ever
• former
• serious