Douglas Harper's Etymology Dictionary
trucial
1876, from truce + -ial. Trucial States, the pre-1971 name of the United Arab Emirates, is attested from 1891, in reference to the 1835 maritime truce between Britain and the Arab sheiks of Oman.
Wiktionary
trucial
a. Of or pertaining to a truce.
Usage examples of "trucial".
There were only three passengers besides myself -an officer of the Trucial Oman Scouts and two oilmen who were straight out from England and could tell me nothing.