Hard

difficile

(requiring a lot of effort to do or understand)

dur

(resistant to pressure)

dur

(requiring a lot of effort to do or understand)

dur

(demanding a lot of effort to endure)

dur

(severe)

dure

(of water, high in dissolved calcium compounds)

fort

(of drink: strong)

irréfutable

(unquestionable)

dur

(with much force or effort (related to act of think))

durement

(with much force or effort (related to act of think))