per·ti·na·cious 
(pûr′tn-āshəs)
(pûr′tn-āshəs)adj.
1. Holding tenaciously or stubbornly to a purpose, opinion, or course of action: a pertinacious heretic; pertinacious defiance.
2. Extremely persistent or unyielding: pertinacious researchers.
[From Latin pertināx, pertināc- : per-, per- + tenāx, tenacious (from tenēre, to hold; see ten- in the Appendix of Indo-European roots).]
per′ti·nacious·ly adv.