Inconsistent means not staying the same, or not fitting together. Inconsistent results, an inconsistent story, inconsistent with the facts.
The two senses are worth keeping apart: a performer is inconsistent over time, and a statement is inconsistent with another at one moment. In law and science, inconsistent evidence is what breaks a case. Note consistency; and that the Latin means standing together, so inconsistent things do not stand together.
1adjectivestayingNot staying the same over time.
“The team's form has been inconsistent, winning one week and losing badly the next.”
2adjectiveinconsistentNot fitting together with something else.
“His story was inconsistent with what the other witnesses said.”
inconsistent is a quality of variation or contradiction.
Latin
consistens
English
consistent
English, today
inconsistent
Built from
From in- + consistent. consistent Borrowed from Latin consistens. Latin consistens, present participle of cōnsistō (“to agree with; to continue”), from con- (ultimately from Proto-Indo-European ḱóm (“beside, by, near, with”)) + sistō (“to cause to stand; to place, set”) (ultimately from Proto-Indo-European stísteh₂ti (“to be standing up; to be getting up”), from the root steh₂- (“to stand (up)”)). by surface analysis, consist + -ent.
Etymology from Wiktionary, CC BY-SA