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?