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1. To approach someone or something slowly and often stealthily. I crept up behind my brother and startled him.
2. To encroach (upon one or something) gradually over time. It wasn't until I sat down that I realized how much exhaustion had crept up on me. The weeds have been really creeping up on the edge of the yard over the last few years.
[for darkness] to move gradually and slowly [toward someone or something]. Dusk crept up and swallowed us in darkness.