wait up

verb

1. To stay awake waiting for somebody to return.

   “Anyway, Dad waited up for them. It must have been about midnight when they came in.”

   “It is one a.m. on a Sunday morning and I'm waiting up for my teenage son, who was supposed to be home at midnight.”

2. To wait.

   “Hey, wait up! I forgot my umbrella.”

   “We have to wait up for Steve.”