Vancomycin

Etymology
Vancomycin:

vanco: "vanquish" (from english "vanquish", meaning "to defeat")

mycin: "fungi" (from greek μύκης (mykes), meaning "fungus")

Explanation
Vancomycin is a "last resort" antibiotic that was created to kill strains of Staphylococcus aureus (MRSA) which were resistant to penicillin-like antibiotics. Think of it as a really tough warrier that is the only one able to "vanquish" (vanco) its foe (MRSA). That's not strictly true anymore, since there are other new drugs that can kill MRSA, however it is still the drug of choice for MRSA. The mycin portion of its name is ironic, since it was isolated from a bacteria, Amycolatopsis orientalis, and not a fungus. However, that bacteria is of the order Actino myce tales, presumably named because of its fungus-like characteristics.