1. To place, insert, or load someone or something inside of something else. In this usage, a noun or pronoun is used between "slip" and "into." Go up to their porch and slip this letter into the mailbox. He slipped a new magazine into the rifle and cocked it.

2. To enter something or some place quickly, easily, or inconspicuously. The teacher had already started his roll call, but Josh managed to slip into the classroom without him noticing. I hate trying to slip into a sleeping bag without waking other people. Let me just get out of this formalwear and slip into some jeans and a tee shirt.