What is yesterday?
yesterday is (adv), (noun) 1. the day before today Yesterday was March 1st so today must be the 2nd. She came to see us yesterday evening. the day before yes- terday two days before today It rained the day before yesterday. The shop only opened the day before yesterday. 2. recent times The government is beginning to look like a group of yesterday’s men. I wasn’t born yesterday I know all about it Of course I know people are smuggling ciga- rettes – I wasn’t born yesterday you know!
source: Easier English, Student Dictionary Upper Intermediate Level