"the largest, most hideous, and most ferocious of the baboons" [OED], 1744, perhaps ultimately from a West African language, but formed into the English components man (n.) + drill (n.4) "baboon," which is of West African origin. The earliest reference reports the name is what the animal was "called by the white men in this country [Sierra Leone], but why it is so called I know not, nor did I ever hear of the Name before, neither can those who call them so tell, except it be for their near Resemblance of a human Creature, though nothing at all like an Ape." [William Smith, "A New Voyage to Guinea"].  French mandrill, Spanish mandril seem to be from English.

*terə-, Proto-Indo-European root meaning "to rub, turn," with derivatives referring to twisting, also to boring, drilling, piercing; and to the rubbing of cereal grain to remove the husks, and thus to threshing.

It forms all or part of: atresia; attorn; attorney; attrition; contour; contrite; detour; detriment; diatribe; drill (v.) "bore a hole;" lithotripsy; return; septentrion; thrash; thread; thresh; throw; threshold; trauma; trepan; tribadism; tribology; tribulation; trite; triticale; triturate; trout; trypsin; tryptophan; turn.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit turah "wounded, hurt;" Greek teirein "to rub, rub away;" Latin terere "to rub, thresh, grind, wear away," tornus "turning lathe;" Old Church Slavonic tiro "to rub;" Lithuanian trinu, trinti "to rub," Old Irish tarathar "borer," Welsh taraw "to strike."

early 14c., "to pierce, penetrate," metathesis of Old English þyrlian "to perforate, pierce," from þyrel "hole" (in Middle English, also "nostril"), from þurh "through" (compare Middle High German dürchel "pierced, perforated;" from PIE root *tere- (2) "cross over, pass through, overcome") + -el. Meaning "give a shivering, exciting feeling" is first recorded 1590s, via metaphoric notion of "pierce with emotion." Related: Thrilled; thrilling.