あく
to become available or vacant
One seat empties and there is a hole where somebody was sitting. The same あく as the train doors going by themselves, except that what opens here is a gap. One free at the back: あく.
Same reading as 開く あく, to open, which is the door rather than the space. The same characters read すく mean a place thinning out.
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Emaki is not affiliated with, or endorsed by, Kaishi 1.5k or its authors. The word, reading and meaning come from the Kaishi 1.5k deck, used with their permission. The mnemonic, the note and the kanji breakdown are original work, published under CC BY-SA 4.0.