Insipidus
Insipidus means “Lacking taste; unpalatable; Lacking flavor or zest; not tasty. E.g. an insipid soup”. Insipidus comes from the Latin word īnsipidus: Latin in-, not; + Latin sapidus, savory (from sapere, to taste)
Diabetes Insipidus
[New Latin diabētēs īnsipidus, literally, insipid diabetes (so called because the urine of those suffering from the disease lacks the sweet taste characteristic of diabetes mellitus): Medieval Latin diabētēs, diabetes; see diabetes + Latin īnsipidus, insipid; see insipid.]
Mellitus
Mellitus comes from the Latin word mellītus which means “honey-sweet”.
Diabetes Mellitus
“[New Latin diabētēs mellītus, literally, honey-sweet diabetes (so called because excessive glucose is excreted in the urine and the resulting sweet taste of the urine was used in diagnosis): Medieval Latin diabētēs, diabetes; see diabetes + Latin mellītus, honey-sweet (Latin mel, mell-, honey;”
Reference
thefreedictionary.com