[LiberLibri]

Spoiler for trivial knowledge:

In the late 19C and early 20C, the expression "zenzen" + positive statement was not regarded as an ugly slang for youngsters. Some great authors at that time employed it quite casually. For example, you witness it in Natsume Souseki's Greenhorn (坊ちゃん).

「一体生徒が全然悪いです。どうしても詫(あや)まらせなくっちゃ、癖になります。退校さしても構いません。」
「私は教頭及びその他諸君のお説には全然不同意であります」

Similar usage can be found in the works of R. Akutagawa, O. Mori, and T. Ishikawa.

It is an interesting point of chronological linguistics; the expression once declined to be obsoleted, and revived again after a century, everybody being unaware of the history.