ἰσχυρίζεσθαι
From LSJ
Greek > English (Woodhouse Verbs Reversed)
(see also ἰσχυρίζομαι): affirm, assert, assert confidently, assert dogmatically, assert emphatically, assert positively, assert vigorously, base one's confidence on, contend that, maintain an opinion, press one's views, protest in argument, take one's stand on, trust to, urge strongly
Lexicon Thucydideum
confirmare, to strengthen, 3.44.3, 4.23.1, 4.68.6, 5.26.3, 6.55.1, 7.49.1, 7.49.4.