Premature
noun, な adjective, の adjective
You already learned that the word 時期 means "time period," so when you add furthermore and early to that, it means that something happened ahead of time. It means premature.
じきしょうそう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
その話を持ち出すのは時期尚早だろう。
I think it’s premature to broach that topic.