"having the same properties in all directions," 1856, from iso- + -tropic, from Greek tropikos "belonging to a turning," from tropos "a turning, way, manner," from trepein "to turn" (from PIE root *trep- "to turn"). Isotropous is from 1859.
before vowels often is-, word-forming element meaning "equal, similar, identical; isometric," from Greek isos "equal to, the same as; equally divided; fair, impartial (of persons); even, level (of ground)," as in isometor "like one's mother." In English used properly only with words of Greek origin; the Latin equivalent is equi- (see equi-).
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit trapate "is ashamed, confused," properly "turns away in shame;" Greek trepein "to turn," tropos "a turn, direction, course," trope "a turning;" Latin trepit "he turns."