〜こととて
since, as (apologetic and formal)
Plain form / Noun + の + こととて
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こととて gives a reason, usually to excuse or apologise for something: 慣れぬこととて、失礼いたしました, "please forgive me, I'm not used to this".
It's old-fashioned and very formal, found in polite speech, letters and period dramas. Classical forms like 慣れぬ (= 慣れない) and 知らぬ often come before it.
The second half is often an apology (失礼しました, お許しください) or an explanation of why something can't be done: 休日のこととて、担当者が不在です, "as it's a holiday, the person in charge is away".
Compare ことから (N2), "from the fact that", which gives neutral, logical grounds. こととて is humble and apologetic.
Examples
慣れぬこととて、失礼いたしました。
Please forgive me, I'm not used to this.
子どものしたこととて、どうかお許しください。
It was only a child who did it, so please forgive them.
休日のこととて、担当者が不在です。
As it's a holiday, the person in charge is away.
急なこととて、何の準備もできておりません。
As it was so sudden, we haven't been able to prepare anything.
初めてのこととて、至らない点もあるかと思います。
As this is my first time, I'm sure there will be shortcomings.