"mutually agreed-upon temporary intermission of hostilities," early 13c., triws, variant of trewes, originally plural of trewe "faith, assurance of faith, covenant, treaty," from Old English treow "faith, truth, fidelity; pledge, promise, agreement, treaty," from Proto-Germanic *treuwo- (source also of Old Frisian triuwe, Middle Dutch trouwe, Dutch trouw, Old High German triuwa, German treue, Gothic triggwa "faith, faithfulness"), from PIE root *deru- "be firm, solid, steadfast." Related to true (adj.). The Germanic word was borrowed into Late Latin as tregua, hence French trève, Italian tregua.
adjectival word-forming element, variant of -al (1) with connective -i-. From Latin -ialis, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective.