I. | ̷ ̷ ̷ ̷| ̷ ̷ transitive verb
Etymology: under (I) + thrust
1. : to thrust underneath or out from underneath
sulky underthrust lower lip
2. : to insert or move (a rock mass) into position by underthrust faulting
II. ˈ ̷ ̷ ̷ ̷ˌ ̷ ̷ noun
1. : the insertion by compressive stresses and faulting of one mass of rock under another — compare thrust fault
2. : the stress that produces an underthrust