oxalylChem.|ˈɒksəlɪl| [f. oxal- + -yl.] The hypothetical radical (C2O2) of oxalic acid. 1859Fownes Man.Chem. 398 One molecule of C4H4 (ethylene) and C4O4 (oxalyl).1873C. H. Ralfe Phys.Chem. p. xxi, Oxalic acid, C2H2O4, is a double molecule of water in which half the hydrogen is replaced by oxalyl.