Yesternight a mix of two common words: Yesterday+Night.
This word is often used to describe a event or something that happened the previous night.
Thus is slang commonly used in Africa…
Person1: Just Yesternight I looked up celebs my age.
Person 2: Thats cool I also looked up celebs my age last night!
The word for yesterday night, or last night.
"Remember what you said yesternight before you left?"