OVER
*In English we use the prefix OVER to mean EXCESSIVELY or COMPLETELY. 

“I can’t believe the team lost the league championship after such a strong season. I guess they thought the final would be easy so they got overconfident.” = excessively (too) confident

“I was overjoyed when I found out that I won a free vacation!” = completely happy

both of these words are adjectives

UNDER
*In English we use the prefix UNDER to mean BELOW or NOT ENOUGH. 

“I’m gonna send this hamburger back. It’s undercooked.” = not cooked enough

“Peter applied for a new job but he was underqualified.” = lacking the qualifications

both of these words are adjectives

“We were overconfident for the game. The challengers played great! Unfortunately we underestimated them and they beat us.”

*Can you guess the meaning of underestimate?