0 n-grams (too new) but a lot of WWW hits.

@@ -11,0 +12 @@
+<misc>&col;</misc>
@@ -13 +14,2 @@
-<gloss>a stylized uterus-shaped mark below a female's navel</gloss>
+<gloss>inmon</gloss>
+<gloss g_type="expl">stylized uterus-shaped mark below a female's navel (usu. in pornography)</gloss>

https://en.wiktionary.org/wiki/淫紋
https://ja.wikipedia.org/wiki/淫紋
(NSFW:) https://www.japanesewithanime.com/2019/04/inmon.html

Wiktionary's usage "on a woman's pubis" is inaccurate, as in Japanese media these tattoos are obviously not exclusive to adult women.
Further, "pubis" refers precisely to the bone situated below the womb, not the skin situated in front of it.
Incidentally, although anatomically accurate "x-ray" depictions of the uterus would lie half-way between the vaginal entrance and the navel, these tattoos are often depicted higher up on the abdomen, even almost touching the belly button (perhaps corresponding to depictions of exaggeratedly large phalluses). 
Hence my phrasing.