Why does いまいち have the meaning of “not good”?

Question

いまいち imaichi (common) 【今一】 いまひとつ imahitotsu (common) · イマイチ IMAICHI 【今一 · 今一つ · いま一つ · 今ひとつ】 adjectival noun → conjugation / adverb:

1.)one more;  another;  the other; —Usually written using kana alone. Colloquialism.

2.)not quite;  not very good;  lacking

いまいち東芝は信用できない。 I can't really trust Toshiba.

いい人だけどイマイチね。 He is nice, but he's not the one for me. ▾ not quite;  not very good;  lacking

Is it because if something needs "one more" then it is lacking in something, and thus can be seen as not good?

Answer 1

You have answered your own question!

いまいち is only the new colloquial form of いまひとつ, which is why it is often written in katakana whereas いまひとつ is always written in hiragana or with kanji such as 今一つ or 今ひとつ.

Additionally, we sometimes say いまに, いまさん, etc. to express greater degrees of "lacking".