Interjection

1. A loud exclamation intended to scare someone, especially a child. Usually used when one has been hidden from the victim and then suddenly appeared unexpectedly.
2. A word used ironically in a situation where one might have scared someone, but said someone was not scared. Not said as loudly as in definition 1.
3. A word used by someone who is inherently scary, such as a ghost or a vampire, to make a weaker person who is scared stiff flee. Usually used quietly, as with definition 2.
4. An exclamation used by a member of an audience, as at a stage play or sports game, to indicate derision or disapproval of what has just occurred.

Noun

1. A derisive shout (as interjection definition 4 above).
2. A close acquaintance or significant other.

Verb

1. To shout extended boos derisively.

   When he took the podium, the crowd booed.

   • 2004, The New Yorker, 18 Oct 2004

     Nobody booed and nobody clapped

2. To derisively shout extended boos at.

   The protesters loudly booed the visiting senator.

Verb

1. I cry aloud, bellow, roar; bray.
   • c. 123-180 AD — Lucius Apuleius Platonicus, Metamorphoses, 7.3

     sed in prima remansi voce et identidem boavi

     but I stayed stuck on the first syllable and brayed it repeatedly

2. I call loudly upon; bellow, cry or roar forth.

Verb

1. To bow, to stoop.
2. To bend, to curve.
3. To make something bend or curve.