A qualifying phrase intended to illustrate that someone has done a great deal in a short time... i.e: between the hours of 9 and 12, the shorter "half" of the workday.
Used sarcastically, sort of a "that's it?" illustrator.
I've researched the Abraham case on WestLaw and Lexis, ordered new office software for the front desk, sat three new client consultations, run to the courthouse and filed our motions, and had my oil changed on the way back, all before lunch.
Sarcastically: Hey, I called the office supplies place and ordered the pens you asked for. "Wow... All that before lunch?"