bo

from The Century Dictionary.

• The earlier word for both.
• An exclamation used to inspire surprise or fright; especially, a cry uttered by children to frighten their fellows. Also boo.

from the GNU version of the Collaborative International Dictionary of English.

• interjection An exclamation used to startle or frighten.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• interjection An exclamation used to startle or frighten.
• noun US, slang Fellow, chap, boy.
• noun martial arts A quarterstaff, especially in an oriental context.