in·de·fat·i·ga·ble 
(ĭn′dĭ-fătĭ-gə-bəl)
(ĭn′dĭ-fătĭ-gə-bəl)adj.
Having or showing a capacity for persistent effort; not tiring or relenting: See Synonyms at tireless.
[Obsolete French indéfatigable, from Latin indēfatīgābilis : in-, not; see IN-1 + dēfatīgāre, to tire out (dē-, intensive pref.; see DE- + fatīgāre, to weary).]
in′de·fat′i·ga·bili·ty n.
in′de·fati·ga·bly adv.