let something slip by
and let something slide by 1. Lit to permit something to move quickly by oneself. He let the ball slip by and he knew he had better get the next one. The careless cashier let the leaky milk carton slide by.
2. Fig. to forget or miss an important time or date. I'm sorry I just let your birthday slip by. I let it slide by accidentally.
3. Fig. to waste a period of time. You wasted the whole day by letting it slip by. We were having fun, and we let the time slide by.