A printer needs a special kind of ink known as toner. Ink toner is put in cartridges. The word "tone" can be found in "toner". The word "tone" is also a verb, and can mean to give strength to a muscle in the body. Toned abs refer to abdominal muscles that are strong from exercise. This joke is playing on the word toned because it sounds close to toner, which can reference printer ink.
The man leaned on the printer cartridge because he wanted to tone up his abs .