What does lēgisdoctor mean in Latin?

lēgisdoctor, a noun, means “doctor/teacher of law; legislator”.

What is the genitive of lēgisdoctor?

The genitive singular is lēgisdoctōris. lēgisdoctor is masculine, third declension.

In classical Latin, [ɫeː.ɡɪsˈdɔk.tɔr]; in ecclesiastical (church) Latin, [le.d͡ʒizˈdɔk.tor].

noun · masculinethird declension

lēgisdoctor m (genitive lēgisdoctōris, feminine lēgisdoctrīx or lēgisdoctorissa)

Classical[ɫeː.ɡɪsˈdɔk.tɔr]Ecclesiastical[le.d͡ʒizˈdɔk.tor]

doctor/teacher of law; legislator
Dē ōrātiōne Dominī ad Patrem, verbīs ad discipulōs, et respōnsō factō lēgis doctōrī, ex eōdem.From the same source, concerning the Lord's prayer to the Father, his words to the disciples, and the response given to the teacher of the law.207 CE – 208 CE, Tertullianus, Adversus Marcionem 4.25
surgēns autem quīdam in conciliō Pharī̆sæus nōmine Gamalihēl lēgis doctor honōrābilis ūniversæ plēbī iussit forās ad breve hominēs fierīThen stood there up one in the council, a Pharisee, named Gamaliel, a doctor of the law, had in reputation among all the people, and commanded to put the apostles forth a little space;Late 4th century, Jerome [et al.], transl., edited by Roger Gryson, Biblia Sacra: Iuxta Vulgatam Versionem (Vulgate), 5th edition, Stuttgart: Deutsche Bibelgesellschaft, published 2007, →ISBN, Actuum Apostolorum 5:34, page 1706, column 1:

declension
form	singular	plural
nominative	lēgisdoctor	lēgisdoctōrēs lēgis doctōrēs
genitive	lēgisdoctōris lēgis doctōris	lēgisdoctōrum lēgis doctōrum
dative	lēgisdoctōrī lēgis doctōrī	lēgisdoctōribus lēgis doctōribus
accusative	lēgisdoctōrem lēgis doctōrem	lēgisdoctōrēs lēgis doctōrēs
ablative	lēgisdoctōre lēgis doctōre	lēgisdoctōribus lēgis doctōribus
vocative	lēgisdoctor	lēgisdoctōrēs lēgis doctōrēs

lēgis-doctor (also written separately), ōris, m.,

I. a doctor or teacher of the law, Tert. adv. Marc. 4, 25; Vulg. Act. 5, 34.

From lēgis + doctor.