In the Edo period, shunga called わ印 were secretly enjoyed.
ワ印
Katakana variant of the first character, matching the reading ワじるし.
〇印
Alternate spelling using a circle symbol; attested but uncommon.
ワじるし
Etymology
Derived from the わ in 笑い絵 (warai-e), an old term for shunga. The exact historical derivation is uncertain, but the spelling is conventionally associated with this meaning.