A: Is he smoking again?
B: He started back up a month ago.

What does "back up" mean? Making some copy?
LcchangWhat does "back up" mean?
You're not associating the right words together.

It's "start up" + "back" ("again") = "start back up" = "start up again" = "start again"

The more central meaning of "back" implies a return. Here "he" is returning to smoking, that is, he is smoking again, as he was before.


come down + 'returning' = come back down

The climbers went up the mountain and came back down.

put (something) in + 'returning' = put (something) back in

Susan took the scissors out of the basket and then put them back in.

He started up again. "Back" means "again" here, sort of.
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