Buck–up

(of a saddle or pack animal) to leap with arched back and come down with head low and forelegs stiff, in order to dislodge a rider or pack.
Informal. to resist or oppose obstinately; object strongly:
The mayor bucked at the school board’s suggestion.
(of a vehicle, motor, or the like) to operate unevenly; move by jerks and bounces.
to throw or attempt to throw (a rider or pack) by bucking.
to force a way through or proceed against (an obstacle):
The plane bucked a strong headwind.
to strike with the head; butt.
to resist or oppose obstinately; object strongly to.
Football. (of a ball-carrier) to charge into (the opponent’s line).
to gamble, play, or take a risk against:
He was bucking the odds when he bought that failing business.
to press a reinforcing device against (the force of a rivet) in order to absorb vibration and increase expansion.
an act of bucking.
buck for, to strive for a promotion or some other advantage:
to buck for a raise.
buck up, to make or become more cheerful, vigorous, etc.:
She knew that with a change of scene she would soon buck up.
Contemporary Examples

Dumped, and Over It Carolyn Sun August 21, 2011
Sarah Palin Rewrites Dr. Seuss Olivia Nuzzi March 8, 2014
Supercommittee Sham: The Deadline and Trigger Farce Michelle Cottle November 17, 2011

Historical Examples

Walter and the Wireless Sara Ware Bassett
The Hero William Somerset Maugham
The Seven-Branched Candlestick Gilbert W. (Gilbert Wolf) Gabriel
The Riddle of the Sands Erskine Childers
The Pirate Shark Elliott Whitney
The Love Affairs of Pixie Mrs George de Horne Vaizey
The Bachelors William Dana Orcutt

verb (adverb) (informal)
to make or cause to make haste
to make or become more cheerful, confident, etc
noun

the male of various animals including the goat, hare, kangaroo, rabbit, and reindeer
(as modifier): a buck antelope

(South African) an antelope or deer of either sex
(US, informal) a young man
(archaic) a robust spirited young man
(archaic) a dandy; fop
the act of bucking
verb
(intransitive) (of a horse or other animal) to jump vertically, with legs stiff and back arched
(transitive) (of a horse, etc) to throw (its rider) by bucking
(informal, mainly US & Canadian) when intr, often foll by against. to resist or oppose obstinately: to buck against change, to buck change
(transitive; usually passive) (informal) to cheer or encourage: I was very bucked at passing the exam
(US & Canadian, informal) (esp of a car) to move forward jerkily; jolt
(US & Canadian) to charge against (something) with the head down; butt
noun
(US & Canadian, Austral, informal) a dollar
(South African, informal) a rand
a fast buck, easily gained money
bang for one’s buck, See bang1 (sense 15)
noun
(gymnastics) a type of vaulting horse
(US & Canadian) a stand for timber during sawing Also called (in Britain and certain other countries) sawhorse
verb
(transitive) (US & Canadian) to cut (a felled or fallen tree) into lengths
noun
(poker) a marker in the jackpot to remind the winner of some obligation when his turn comes to deal
(informal) pass the buck, to shift blame or responsibility onto another
(informal) the buck stops here, the ultimate responsibility lies here
noun
Pearl S(ydenstricker). 1892–1973, US novelist, noted particularly for her novel of Chinese life The Good Earth (1931): Nobel prize for literature 1938
n.

The ‘buck’ is any inanimate object, usually knife or pencil, which is thrown into a jack pot and temporarily taken by the winner of the pot. Whenever the deal reaches the holder of the ‘buck’, a new jack pot must be made. [J.W. Keller, “Draw Poker,” 1887]

Perhaps originally especially a buck-handled knife. The figurative sense of “shift responsibility” is first recorded 1912. Buck private is recorded by 1870s, of uncertain signification.
v.

A dollar (1850s+)
A hundred dollars, esp as a bet (1960s+ Gambling)
buck private
A Roman Catholic priest (1920s+ Hoboes)
A young male Indian; Native American brave (1800+)
young black man (1830s+)
Any young man, esp a strong and spirited one; bucko (mid-1700s+)

To resist; defy; go up against •Often in the negative: You can’t buck the system/ Life is a combination hard to buck, A proposition difficult to beat (1850s+)
To work for personal advancement; aspire eagerly; covet: I’m bucking for that dealership (1880s+)
To pass along a letter, memorandum, problem, etc, usually without taking action; pass the buck: Let’s buck this one to the Committee on Hot Potatoes (WWII armed forces)

Cheer up, become encouraged, as in Buck up! We’ll soon have it done, or Even the promise of a vacation did not buck her up. This term was first recorded in 1844.

buck for
buck stops here, the
buck up