upstart

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person of humble origin who attains sudden wealth, power, or importance, especially one made immodest or presumptuous by the change; a parvenu.
• adjective Suddenly raised to a position of consequence.
• adjective Self-important; presumptuous.
• intransitive verb To spring or start up suddenly.

from The Century Dictionary.

• To start or spring up suddenly.
• noun One who or that which starts or springs up suddenly; specifically, a person who suddenly rises from a humble position to wealth, power, or consequence; a parvenu.
• noun One who assumes a lofty or arrogant tone.
• noun A puddle made by the hoofs of horses in clayey ground. Halliwell. The meadow-saffron, Colchicum autumnale, whose flowers spring up suddenly without leaves.
• Starting up suddenly; quickly rising.
• Suddenly raised to prominence or consequence; parvenu: as, “a race of upstart creatures,”
• Characteristic of a parvenu; new and pretentious.

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

• intransitive verb To start or spring up suddenly.
• adjective Suddenly raised to prominence or consequence.
• noun One who has risen suddenly, as from low life to wealth, power, or honor; a parvenu.
• noun (Bot.) The meadow saffron.

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

• noun One who has suddenly gained wealth, power, or other prominence, but either has not received social acceptance or has become arrogant or presumptuous.
• adjective self-important and presumptuous
• verb to rise suddenly, to spring

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an arrogant or presumptuous person
• noun a gymnastic exercise performed starting from a position with the legs over the upper body and moving to an erect position by arching the back and swinging the legs out and down while forcing the chest upright
• noun a person who has suddenly risen to a higher economic status but has not gained social acceptance of others in that class
• adjective characteristic of someone who has risen economically or socially but lacks the social skills appropriate for this new position