Definition of arch verb from the Oxford Advanced Learner's Dictionary

arch

verb
ɑːtʃ
 
; ɑːrtʃ
 
 
1 [transitive, intransitive] arch (something) if you arch part of your body, or if it arches, it moves and forms a curved shapeThe cat arched its back and hissed.2 [intransitive] to be in a curved line or shape across or over somethingTall trees arched over the path.