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