1 to serve

2 to wait upon

1 servo (small electric motor)

2 computer server

1 to wait for an opportunity

2 to watch for one's chance

1 to spy upon

2 to lie in wait for (an opportunity)

1 lit. to look for holes, and observe gaps (idiom)

2 fig. to search out one's opponent's weak points

3 to look for the Achilles' heel