We found one dictionary that defines the word know what's o'clock:

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1. know what's o'clock: Wiktionary

Definitions from Wiktionary (know what's o'clock) ▸ verb:  (dated) Synonym of know what is what ▸ Words similar to know what's o'clock ▸ Usage examples for know what's o'clock ▸ Idioms related to know what's o'clock ▸ Wikipedia articles (New!) ▸ Words that often appear near know what's o'clock ▸ Rhymes of know what's o'clock ▸ Invented words related to know what's o'clock