Meaning of K-ON title

Seen this off-hand remark by Sixten:

[] what is more interesting is [Mr. Watanabe's] stated occupation, “K-On”. Apparently, in addition to being a high-level Magic player, he is a professional musician who plays in a light music (軽音楽, “keiongaku”, or “kei-on” for short) band.

Why, I didn't know that. Reminds me of a russian proverb: "век живи — век учись" and its vulgar derivative "век живи, век учись, дураком помрешь".

UPDATE: Omo says that "kei-" does not mean "light" music, but "anything that’s non-classical and non-traditional." So, if I understand right, heavy metal, punk rock, and rap are all included (says he, anyway).

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