1. To approach someone or something in a quick, sneaky, and furtive manner so as to reach them or it without being noticed. Someone managed to slip up on Sneaky Pete, which I thought was all but impossible! The spy slipped up on the guard tower and cut the power to the lights and security cameras.

2. To happen to or unfold before someone in a very quick, imperceptible manner. Old age has slipped up on me. Where have all the years gone? It feels like the gentrification of this town has been slipping up on those of us who've lived here our whole lives.

3. To make a foolish blunder or mistake in, about, or regarding something. I think I slipped up on the address when I was shipping your package. Wow, it looks like the accountants must have really slipped up on the tax return.