一々(いちいち)one-by-one, separately, every single, each and every, without omission, fully, in detail

子供(こども)が親(おや)に：「いちいちうるさいなあ！」

Child to his parents: 'You're so naggy every single time!'

分(わ)からない単語(たんご)を全(すべ)て一々(いちいち)辞書(じしょ)で調(しら)べました。

I looked up every single word I didn't know in the dictionary.

先生(せんせい)：「一々(いちいち)説明(せつめい)はしないから、ちゃんとメモをとりなさい！」

Teacher: 'I won't explain everything in detail, so make sure to take notes!'

僕(ぼく)の彼女(かのじょ)はメンヘラで一々(いちいち)構(かま)ってくる。その上(うえ)、浮気(うわき)症(しょう)なので、別(わか)れるつもりだ。

My girlfriend is a nutjob who always fully needs my attention. Moreover, she's a cheater, so I'm planning to break up with her.

クレーマーというのは、こちらに非(ひ)が無(な)くても一々(いちいち)難癖(なんくせ)をつけてくるものです。

A complainer is someone who constantly finds fault with each and every thing, even when there is no wrongdoing on our part.