Should you, after reading the title, now be questioning your basic grasp of Japanese, goal achieved. But seriously, I’m not kidding. Didn’t you ever wonder about this word, after having looked up the kanji used to write nigate, which only means unskilled nowadays, and finding that niga(i) is the word for bitter? I sure did.

The truth is, the original usage of this word was perfectly understandable. But let’s make this another episode of let’s read Japanese dictionary and consult the equivalent of the Bible for the linguist/ambitious student:

広辞苑第六版
にが-て【苦手】
①爪がにがく、手に毒のあるという手。その手でさわると癪(シャク)や腹痛などが治るという。好色一代女(2)「私の一薬なりと、夜明け方までさすりける」

精選版日本国語大辞典
〔名〕
①(「にがで」とも)常人と異なり、不思議な力を持つ手。その手で押さえると人は腹痛が治まり、ヘビは動けずにと捕えられるなどという。
*排諧・底抜磨(1646)上「をさあひ(幼児)の腹をさするは苦手にて」
*浮世草子・世間娘容気(1717)三「天性苦手(ニガテ)といふものにて、小児の虫痞(むしつかへ)をさするに妙を得て」

To summarize, as the heading of this post already states, a bitter poisonous substance was somehow thought to be connected with unusual powers. And one thing we learn about reading classic dictionaries is that we can ignore the examples. They’re mostly from ancient Japanese and are hardly understandable, and they don’t reflect current usage. Use a good dictionary such as 明鏡国語辞典 for that.

I suppose now would be the time for me to tell how we get a 180° flip in meaning to arrive from “super skill” at “no skill”, but alas, I’ve got no idea and I didn’t find anything on the net. Well, I could certainly speculate and make up my own theory, but I’d rather not to that without any evidence to back it up; and it may lead some people to think of me an authority and lead them to take my hypothesis for a fact. Not what I want.

With that being said, I’d love to hear you personal theory in the comments, feel free to speculate.