였겠죠

must have been, was probably

→ 이다

였 Past tense + 겠 conjecture or future + 죠 Polite sentence-ending (confirmation)

auxiliary · informal Rare #30,292

must have been, was probably Expresses a likely guess about the past, often inviting agreement.

KO: 그때는 정말 힘든 시기였겠죠.

EN: It must have been a really difficult time then.

Conjugation of 이다

We were friends, right?

— 맨 프럼 어스 (2007)

It must have been an echo.

— 필사의 추적 (1981)