The opposite is ambitious, which can mean motivated, determined, or can have the connotation of ruthlessness.
nothing
principal
Unambitious
unambitious
Lazy or unmotivated (unambitious).
The opposite could be insincere, or (for enthusiastic) unenthusiastic or unambitious.
unambitious
Examples of words ending with 'tious' are:ambitiouscautiousconscientiouscontentiousdissentiousexpeditiousfacetiousfictitiousflirtatiousinfectiousnutritiousostentatiouspretentiousrambunctiousrepetitiousscrumptiousseditioussurreptitiousvexatious
The closest word I can think of is "unambitious."
The antonym of the word 'synonym' is 'antonym'.
An antonym means "opposite". So an antonym for the word synonym is antonym.