"Thorax" means not only torso as you may probably know, but also the armour which protects it. "Thorakizo" therefore means "to get armoured" or "to get dressed with the armour" and that's where Thorakites (sg.), Thorakitai (pl.) surely comes from.
Iphikratous is the genitive sg. of the general's and mercenary leader's name "Iphikrates", so it means "of Iphikrates". Iphikratous Hoplitai could be translated as hoplites according to Iphikrates.
I just saw that even shoes for soldiers were named Iphikratides.