1. To allow someone or something to quickly pass by one. A noun or pronoun is used between "slip" and "through." Don't let anyone through. 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 through already!

2. To allow someone or something make their or its way through a crowd of other people or things. A noun or pronoun is used between "slip" and "through." Excuse me, please let me through, I need to get to the back of the auditorium. Make sure to always pull over to let emergency vehicles through when you're in traffic.

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