go under

1.  Suffer defeat or destruction; fail. For example, We feared the business would go under after the founder died. [Mid-1800s]
2.  Lose consciousness. For example, Ether was the first anesthetic to make patients go under quickly and completely. This usage dates from the 1930s.
3.  Submerge, sink, as in This leaky boat is about to go under.

Definition: fail, submerge
Antonyms: accomplish, achieve, succeed

The verb has 3 meanings:

Meaning #1: go under, "The raft sank and its occupants drowned"
  Synonyms: sink, settle, go down

Meaning #2: disappear beyond the horizon; of celestial bodies such as the sun and the moon
  Synonyms: set, go down

Meaning #3: be called; go by a certain name
  Synonym: go by