Blend of 萌 (cute, from Japanese 萌え moe) + 新 (new). The 萌 softens the 'newbie' label into something endearing rather than insulting - contrast with the blunter 菜鸟.
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Bilibili / gaming / anime community slang. Often self-deprecating: 我是萌新 = 'I'm new here, be gentle'.
