What does 漫画 Manga mean?

Manga is the ordinary Japanese word for comics. Not Japanese comics: comics. A translated Belgian album on a Japanese shelf is manga, and no qualifier is needed to make that true.

The characters

漫 (man) carries a sense of the rambling, the aimless, the done-without-a-plan. 画 (ga) is a picture or a drawing. The usual gloss is something like whimsical pictures or sketches made freely, and the first character is the interesting one, because it describes a manner of working rather than a subject.

The Hokusai claim, which is almost right

It is repeated everywhere that Katsushika Hokusai coined the word in 1814, for the sketchbooks published under that title. The date and the book are real. The coinage is not.

The compound existed in Chinese before Japanese took it up, where it referred to informal, spontaneous sketches. Hokusai's usage meant roughly that: loose drawings, unconnected to each other, closer to a study collection than to anything with a narrative. The modern sense, sequential comics with panels and stories, is a twentieth-century development and is not what the 1814 volumes are.

So the honest version is that Hokusai used an existing word for a thing that is not what the word now means, and the title later became famous enough that people reasoned backwards. The British Museum has hedged the question in public; the popular sources rarely do.

Why the wrong version is attractive

It supplies an origin story with a great artist at the front of it, and it makes a modern popular medium look continuously old. Both are appealing and neither requires checking, which is roughly the profile of every widely repeated etymology.

The accurate version is better anyway. A borrowed Chinese compound for casual sketching, applied by a printmaker to studies, then repurposed generations later for a mass medium, is a more interesting history than a single act of invention.

The narrowing in English

English borrowed the word for Japanese comics specifically, exactly as it did with anime. Both terms are general in Japanese and specific in English, and both acquired that specificity from the circumstances of the borrowing rather than from anything in the words.

The practical effect is a phrase English cannot say cleanly. Japanese has an unmarked word for the medium and marks the foreign case; English marks the Japanese case and has comics for the rest, so translating a Japanese sentence about comics in general forces a choice the original did not make.