Free dictionary online: translation in English, Spanish, French, Italian, Chinese, Russian, definitions, synonyms...
Results found in several dictionaries: Collins, Collaborative Dictionary
buck up 
  vi 
  (=cheer up) reprendre du courage 
come on, buck up!  allez, courage! 
  (=do better) se remuer 
If we don't buck up we'll be in trouble.   
the Aussies, having been told to buck up, scored a lovely try   
  vt 
  (=cheer up) remonter le moral de 
Anything anybody said to try and buck him up wouldn't sink in.   
The aim was to buck up their spirits in the face of the recession.   
 to buck one's ideas up   (=pull one's sock up) se reprendre 
You'll have to buck your ideas up if you seriously want that promotion.   

Collaborative Dictionary     English-French
pass the buck exp. éviter de prendre une decision; passer la responsabilité, se renvoyer la balle, se refiler la patate chaude (FAMILIER)  

"Collins English French Electronic Dictionary © HarperCollins Publishers 2005"