Rival, Enemy
Noun
たいてき
分かっていると思うけど、油断大敵だよ。「猿も木から落ちる」って言うだろう？
As you all know, overconfidence is man’s greatest enemy. People say "even monkeys fall from trees," right?
Your big enemy is your worst enemy. Your arch enemy.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.