COCK - this is another one of those triflin' kanji. . . the kind where almost every jukugo is pronounced one way, but the ONE JUKUGO that is pronounced the 'wrong way'. . .that's the only one anyone uses!
Yesterday I made a key and stuck it in my SACK.
昨 (yesterday) + 日 (sun, day)
= 昨日 (yesterday)
last - xxx
昨 (yesterday) + (the beautiful sights of xxx)
= 昨 XXX (last - xxx)
last month, last year, etc.