cotangent (n.)youdaoicibaDictYouDict
from co. tangent, abbreviation of complement + tangent (n.).
tangent (adj.)youdaoicibaDictYouDict
1590s, "meeting at a point without intersecting," from Latin tangentem (nominative tangens), present participle of tangere "to touch," from PIE root *tag- "to touch, to handle; border on; taste, partake of; strike, hit;" figuratively "affect, impress; trick, cheat; mention, speak of" (cognates: Latin tactus "touch;" Greek tassein "to arrange," tetagon "having seized;" Old English þaccian "stroke, strike gently"). First used by Danish mathematician Thomas Fincke in "Geomietria Rotundi" (1583). Extended sense of "slightly connected with a subject" is first recorded 1825. Related: Tangence; tangency.
tangent (n.)youdaoicibaDictYouDict
1590s as a geometric function, from tangent (adj.). From 1650s as "a tangent line." Figurative use of off on a tangent is from 1771.
tangental (adj.)youdaoicibaDictYouDict
1742, from tangent (adj.) + -al (1). Related: Tangentally.
tangential (adj.)youdaoicibaDictYouDict
1620s, see tangent (adj.) + -ial. Figurative sense of "divergent, erratic" is from 1787; that of "slightly connected" is from 1825. Related: Tangentially.