Often, people in my culture try to solicit funds for a worthy (and sometimes, not so worthy) cause. Several decades ago, companies began to advise their employees to donate a part of their income to some worthy causes through an automatic deduction from their pay, which was then forwarded to the charity (the worthy cause) by the company. Following that time period, people began to say to solicitors, "I gave at the office", meaning, "I have already donated to a worthy cause through my company". It is commonly used as a polite way of saying, "Don't ask me for any more money because I have already donated."