a conjunction/particle attached after the attributive (plain) form of a verb or adjective to indicate cause or reason; often carries an apologetic or explanatory nuance.
急に熱が出たもので、会議を欠席しました。
I suddenly developed a fever, so I missed the meeting.
忙しいもので返信が遅くなってすみません。
Sorry my reply was late; I've been busy.
急な仕事が入ったもんで、行けなくなったよ。
Some urgent work came up, so I couldn't go (colloquial もんで).
雨が激しいもので、外出は控えたほうがいい。
Since the rain is heavy, it's better to refrain from going out.