Provenance — where it came from
A 沼 (numa) is a swamp or marsh — soft ground that takes hold of a boot and does not give it back. In the 2010s, Japanese fan communities found in it the perfect map of their own experience: you do not join a fandom, you step in the wrong place — and sink.
The grammar of the swamp is precise. One “falls into” it: 沼にハマる (numa ni hamaru). Any passion can be a swamp, and each gets its name by attachment: an idol swamp, a camera-lens swamp, a stage-play swamp, a fountain-pen swamp. The deepest ones are 底なし沼 (sokonashi-numa) — bottomless. Money is understood to be the first thing the swamp digests; fans compare depths by damage to their wallets, with dark satisfaction rather than regret.
The word pairs naturally with 推し (oshi, № 013): the oshi is who you love; the numa is where loving them has left you. Together they form the geography of 2010s–2020s fan life. Older Japanese had many words for devotion. This one is distinct in its honesty about the physics: nobody climbs out, and nobody in the swamp is asking to be rescued.
Field notes — how it’s used
万年筆沼に落ちて3年になる
mannenhitsu-numa ni ochite san-nen ni naru
“It’s been three years since I fell into the fountain-pen swamp.” — a status report, not a cry for help.