prep

• (figuratively) In the face of; in response to (some attacking force).
  "England's World Cup dreams fell apart under a French onslaught on a night when their shortcomings were brutally exposed at the quarter-final stage."

adv

• (usually in compounds) Less than what is necessary to be adequate or suitable; insufficient.
  "The plants were underwatered."
• (informal) In or into an unconscious state.
  "It took the hypnotist several minutes to make his subject go under."

adj

• (medicine, colloquial) Under anesthesia, especially general anesthesia; sedated.
  "Ensure the patient is sufficiently under."
• (informal) Having a particular property that is low, especially so as to be insufficient or lacking in a particular respect.
  "This chicken is a bit under. (insufficiently cooked)"

noun

• (informal) Something having a particular property that is low or too low.
  "I went fishing but caught nothing but unders."
• (gambling) A bet that a particular sporting statistic, such as points scored in a game, will be below a certain stated value.