Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
†† burdo, ōnis (collat. form burdus, i, Acron. ad Hor. C. 3, 27, 7), m., = [??], a mule (the offspring of a horse and she-ass, while mulus is the offspring of an ass and a mare; v. Isid. Orig. 12, 1, 61), esp. used for carrying litters, Dig. 32, 49: onus duorum burdonum, Vulg. 4 Reg. 5, 17.