Why was "tantrum" trending last week?
1886, from macro- + molecule. Apparently coined in "On Macro-molecules, with the Determinations of the Form of Some of Them," by Anglo-Irish physicist G. Johnstone Stoney (1826–1911). Originally of crystals. Meaning "molecule composed of many atoms" is from 1935, from German makromolekul (1922). Related: Macromolecular.
macromolecule mac·ro·mol·e·cule (māk'rō-mŏl'ĭ-kyōōl')
A very large molecule, such as a protein, consisting of many smaller structural units linked together.