もっともらしいじゃなかったです
尤もらしいじゃなかったです
mottomorashiijanakattadesu

Desu Copula
i-Adjective

The polite, past negative form of もっともらしい

1. It didn't seem likely.; It didn't seem plausible.; It wasn't convincing.

Past/Perfective
Negative
Polite
Casual Register
Usually Written in Kana