1. To allow someone or something to quickly pass by one. A noun or pronoun is used between "slip" and "by." Don't let anyone slip by. We don't want the scene of the crime to be contaminated before the detectives have a chance to look things over. Their goalkeeper is not in his usual form today—that's the fourth goal he's let slip by already!

2. To fail to detect someone or something. A noun or pronoun is used between "slip" and "by." She has an eagle eye for mistakes—she hasn't let a single error slip by since she started proofreading for us. The inattentive security guard kept letting prisoners slip by into the storage room where contraband is stored.

3. To fail to remember or pay attention something one had planned or scheduled. A noun or pronoun is used between "slip" and "by." I can't believe you let our meeting slip by—we've had that planned out for over a month! She lets everything else in his life slip by when she gets so focused on a project like this.

4. To allow a period of time to pass or be spent idly or without notice. A noun or pronoun is used between "slip" and "by." I don't want to let the whole summer slip by without going anywhere nice for a vacation. Sorry, I got distracted playing my game and ended up letting the afternoon slip by.

5. To fail to capitalize on some opportunity. A noun or pronoun is used between "slip" and "by." What are you waiting for, call them back! Don't let such an amazing chance slip by!