Where does “青短” come from?

青短 (Japanese) comes from Japanese 短冊, from Japanese 短, from Middle Chinese 短 — short.

青短 (Japanese): Any of the three hanafuda cards that depict a purple or blue tanzaku; i.e. the peony tanzaku card, the chrysanthemum tanzaku card, or the maple tanzaku card

Definitions

Any of the three hanafuda cards that depict a purple or blue tanzaku; i.e. the peony tanzaku card, the chrysanthemum tanzaku card, or the maple tanzaku card

Ancestry of “青短”, step by step

青短 traces back through two separate lines of ancestry.

via Japanese 短冊

Step	Language	Word	Meaning
1	Japanese	短冊	tanzaku; A small strip of paper that can be written on, especially those hung on bamboo or other trees during Tanabata
2	Japanese	短	fault, weak point; minor
3	Middle Chinese	短	short

via Japanese 青

Step	Language	Word	Meaning
1	Japanese	青	blue
2	Middle Chinese	青	—

Every word from Middle Chinese 短 →Every word from Japanese 短 →Every word from Japanese 短冊 →