expression
Follows the te-form of a verb or a noun, indicating that a state or action has continued from that point onward. Often used when the result is a lasting change or ongoing situation.
彼女に会ってからというもの、毎日が楽しい。
Ever since I met her, every day has been enjoyable.
新しい仕事を始めてからというもの、忙しくて休む暇がない。
Ever since I started my new job, I've been so busy I have no time to rest.
Combination of the particle から ('from/since'), the quotative と, the verb 言う ('say'), and the noun もの ('thing'). Literally 'from the thing called...', it grammaticalized into a set phrase meaning 'ever since'.