Definition of yestermorning in English:

yestermorning

Pronunciation /ˈjɛstəˌmɔːnɪŋ/

adverb

literary
  • On or during the morning of yesterday; yesterday morning.

noun

literary
  • The morning of yesterday.

Origin

Late 16th century; earliest use found in Philip Sidney (1554–1586), author and courtier. From yester- + morning.