1. To manage to cross some real or imagined border in a surreptitious or inconspicuous manner; to avoid or evade someone or something while crossing (something). The criminal slipped across the US-Canada border undetected. He slipped across the other team's defensive line and shot the ball into the corner of the net.
2. To sneak or smuggle something across (some place or border) in a surreptitious or inconspicuous manner. The man was arrested for trying to slip drugs across the border. She slipped $200 across the table, picked up the unmarked suitcase, and calmly walked out of the store.