Straight Line
noun
You learned 文字 means "letter (of the alphabet)" or "character." And what does the kanji character for one look like in Japanese? A straight line!
Think of this word as meaning "the 一 character." In Japanese, it's most often used to describe the straight line a person's lips make when they're pressed together in concentration, frustration, or silence.
This is a jukugo word, which usually means on'yomi readings from the kanji. Since 文 has two on'yomi readings, here's a mnemonic to help you remember which one to use:
Anyone who goes around with his mouth in a straight line is probably a monster (もん). How could anyone walk around with such an uninviting expression on their face? Basically, anyone who isn't smiling constantly is a monster, as far you're concerned. And that includes those monsters with 一文字-shaped mouths.
一文字にする
to make a straight line
一文字にむすぶ
to purse (one's lips)
一文字にならぶ
to stand in a straight line
女の子は、おこって口を一文字にむすびました。
The girl got angry and her mouth pursed into a straight line.
サーモンに一文字の切り目が入ってます。
The salmon has a straight line cut across it.
たくさんのドローンが一文字になってとんでいる。
Quite a few drones were flying in a straight line.