1. To force something in some space or crevice. A noun or pronoun can used between "press" and "in." He pressed the paste in the cracks to seal them up. She found a whole beneath the chair where she pressed in her chewed-up gum.
2. To depress something. A noun or pronoun can used between "press" and "in." I pressed in the button, but nothing happened. Make sure you press it in all the way, or it won't work.
See also: press