Muttering Kanji

It means muttering or murmuring, usually to yourself.

In Japanese, 呟 appears in つぶやく, a verb for saying something in a small voice, sometimes with a grumbling edge. The first example cited by a historical Japanese dictionary is from 913, in 亭子院歌合, a Japanese poetry contest.

The classical Chinese dictionary tradition also glosses 呟 as a sound and cites loud vocal noise. The Chinese and Japanese dictionary senses differ in emphasis: one includes loud vocal noise; the Japanese verb centers on quiet self-talk.

Is 呟 a tweet?

Only in a Japanese social-media context.

Japanese dictionaries also give つぶやく the meaning “to tweet.” Japan's early Twitter interface called posts つぶやき; in 2009, it changed the label to ツイート, in line with Twitter's English branding.

This makes “post” a possible reading in a Japanese social-media sentence. In dialogue, fiction, or a complaint, “mutter” or “murmur” is safer.

How do I write or type 呟?

Start with the reading つぶやく, then convert it to kanji.

Japanese input starts from the word's reading, not a dedicated key for 呟. The hiragana form つぶやく also fits casual online Japanese; the kanji form 呟く looks more visibly written.

The whole word carries the grammar: 呟く is a verb, while 呟き is a noun for a mutter, aside, or post.