The Collaborative International Dictionary
Calcitrant
Calcitrant \Cal"ci*trant\, a. [L. calcitrans, p. pr. of calcitrare to kick, fr. calx, calcis, heel.] Kicking. Hence: Stubborn; refractory.
Douglas Harper's Etymology Dictionary
calcitrant
1866, as if from Latin calcitrantem (nominative calcitrans) "kicking" (see recalcitrant). Pedantic humor; probably a back-formation.
Wiktionary
calcitrant
a. stubborn; refractory