Definition:
To be told to pull up your socks is to be told to do a better job on something. It is a negative review of one's performance. It can also be directed at someone who generally needs to pull himself together, often to face a difficult or unpleasant task.

Usage:
"If you don't pull up your socks on the Reitman contract, you'll be fired."

Explanation:
Perhaps this expression is related in origin to the military slang "drop your cocks and grab your socks", which J.E. Lighter's Historical Dictionary of American Slang (Random House, 1994) identifies as nautical/military slang meaning "wake up!" (addressed to sleepers in a forecastle or barracks). Lighter has citations as early as the 1920s. Much more recently (May 13, 2005), US comedian Bill Maher used it in a monologue on his "Real Time With Bill Maher" program:

Trivia:
This expression is unlikely to be heard or understood in the United States. US synonyms include shape up, get it together, pull yourself together, cle ...more