Deadly, deadly dull

At a toy factory

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Worker1: Why are you yawning? Are you tired? You should have gone to
bed early. If you had gone to bed early, then you wouldn't be yawning now.
Worker2: I'm not yawning because I'm tired. I'm yawning because I'm bored.
Worker1: Bored?
Worker2: Yes. It's boring putting these blocks into these boxes day after day,
month after month, year after year. It's boring. It's dull. It's deadly, deadly dull.

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What does "deadly, deadly dull" mean?

Does it mean "very, very dull" ?

Thank you