You are right.
尻川 and 骹川 are both commonly used.
尻，according to 《廣韻》 system, belongs to (溪)-onset (豪)-rime (平)-tone
(一等)-rime grade (開口韻)-rime type, with a (反切)-fanqie = 苦刀
骹，according to 《廣韻》 system, belongs to (溪)-onset (肴)-rime (平)-tone
(二等)-rime grade (開口韻)-rime type, with a (反切)-fanqie = 口交
Phonetically speaking, 骹 would be a better candidate because
it would yield kha1 rather than 尻
which would yield kho1 in Contemporary
Hokkien
◎ However, 尻 can become the exception to the
rule (i.e. evolve into kha1) only when we
reflect on the case of 早, which also belongs to the (豪)-rime
and (一等)-rime grade, and evolved into ca2 in Contemporary Hokkien instead of the
expected co2
◎ These
unexpected forms sometimes may occur in descendant languages due to
missing entries for alternate forms of pronunciation in the old
rime books
Semantically speaking, 尻 would be a better candidate because
it means "end of spine;
buttocks; sacrum" ::reference http://chinese.dsturgeon.net/dictionary.pl?if=gb&id=31848::
In Modern Japanese, the native word for "buttocks" is しり
(shiri) which utilizes either one of the two
Chinese characters to represent its meaning, 尻 or 臀 respectively.
It is apparent that the meaning of 臀 is also "buttocks"
::reference http://chinese.dsturgeon.net/dictionary.pl?if=en&char=臀::
Etymologically speaking, 川 symbolises the valley-like
curvature of the buttocks, so 尻川 and 骹川 can be understood as
"the valley-like
curvature of the buttocks" and "the valley-like curvature between the
thighs" respectively, depending on which one you like
Side note:
There are currently 4 languages of the 閩 family that I know of that
use k(h)a-川 choice for "buttocks"
閩-閩南-福建閩南
kha1-chng1 (廈門 and 泉州
variety) /
kha1-chuiN1 (漳州 variety)
閩-閩南-潮州閩南
kha1-chng1 (潮州 variety) /
kha1-chɯng1 (汕頭
variety)
閩-閩南-海南閩南
ka1-sui1 (文昌 and 海口 variety)
閩-莆仙
kɒ1-ɬuiN1
(仙游 variety) /
kɒ1-ɬuai1 (莆田
variety)