Picture someone out and calling: who are you, and what are you doing here? That call is the challenge. Unchallenged is the word for what happens when it never comes. Nobody steps out, nothing is asked, and whatever was walking past just keeps walking.
The packet gives the word as un- plus challenged, with un- coming down through Old English from a very old negative element. So the word is built, not borrowed: take the act of challenging and cross it out.
來源把這個字拆成 un-（不）加 challenged，而 un- 這個否定的成分一路從古英語往上追得很遠。所以它是拼出來的，不是借來的：把「挑戰、喝問」這個動作畫掉，剩下的就是這個字。
Wiktionary etymology tree supplied in the evidence packet (source_ref kaikki:3f6f69a190036ae4:872410615): un- + challenged.
From the picture
沒有人出聲反對
擋你的人換成了反駁你的人：該有人說「等一下，這樣對嗎」，結果房間一片安靜。
adjective · B2
Accepted without anyone saying it is wrong. Used about a statement, a claim, an idea or a decision that nobody argues with or asks questions about — very often in the pattern 'go unchallenged'.
沒人質疑的
沒有人給你難題
擋路的那個東西換成一道難題：本來會逼你使力的事情一直沒出現，你也就一直沒被驗過。
adjective · C1
Never given anything hard enough to stretch you, so what you can really do stays untested. Used about a student, a worker or a team for whom everything has been easy.
沒被考驗到的
adjective · B2
Able to go somewhere without being stopped and asked who you are or what you are doing. Used about people entering or moving through a place, often with 'go', 'walk', 'enter' or 'pass'.
沒被攔下盤問的
adjective · B2
Having no rival: nobody is competing for the place, and in a game nobody is close enough to stop you. Used about a leader, a champion, a record, or a player who takes the ball with no opponent near.
沒有對手的