Everybody knows
Under is below
Dog is a dog
Then you know
“underdog”
What does that mean?
Let's learn together.

What does "underdog" mean?
Let's first look at the British-English explanation, a competitor thought to have little chance of winning a fight or contest, the party in the struggle or competition with no hope of winning, so the meaning of "underdog" means: the one who is not optimistic in the game; Weak side.
Example:
capone was an underdog hero, a poor boy who made it big.
Capone was a reckless hero, a poor boy who soared.
most of the crowd were cheering for the underdog to win just this one time.
Most of the people in the crowd were cheering on the underdog to win a race.
"Under the table" means more than just "under the table"
In addition to the meaning of "under the table", "under the table" also has an extended meaning, that is, to mean: secretly; clandestinely.
i had to bend double to get under the table.
I had to hunch my back to get under the table.
they're doing it under the table.
They were doing it in secret.
Go under ≠ go down
In fact, the meaning of "go under" means: bankruptcy; Sunken; succumb.
the firm will go under unless business improves.
If business doesn't pick up, the company will go out of business.
the point man for denver's new airport says there's no way it'll go under.
The head of Denver's new airport said the airport project would never be suspended.