this sounds like a
variant expression for avaruddhā (strī) / avaruddhastrī.
The latter is explained in Olivelle's
Dictionary (2015) as a "mistress or concubine in the
exclusive keeping of one individual until his
death". Avaruddhā is an old and quite common term
for women kept in a harem (avarodha), testified as
orodhā also in Pāli texts.
Closest to your "dhṛtastrī" seems to come "dhṛtā" ("a concubine"). Evidence
to be found in Jaina texts: B.
J. Sandesara, Lexicographical Studies in „Jaina
Sanskrit“. Journal of the Oriental Institute Baroda
9.4 (1960): 60.
A cheerful read:
David Smith, „One
man and many women: some notes on the harem in mainly
ancient and medieval
India from sundry perspectives.“ In: Cracow
Indological Studies 14 (2012), S.
1-16.
Best wishes,
Walter