틀림없이 + guess/future: 틀림없이 올 거예요
surely, without a doubt, certainly
Example
그 사람은 틀림없이 약속 시간에 올 거예요.
He'll definitely come on time.
Word parts
From 틀리다 (to be wrong) + 없이 (without): 'without being wrong'.
Good to know
Pronounced [틀리멉씨] — the ㅆ sound comes from the ㅄ batchim.