Final answer:
The antonym for 'insincere' is 'sincere', which describes someone who is honest and genuine in their expressions and actions, reflecting true feelings and thoughts.
Step-by-step explanation:
The antonym of insincere is sincere. When someone is insincere, they are not genuine or honest in their feelings or expressions. Conversely, a sincere person is honest, genuine, and true in their words and actions. They speak and act with a sense of integrity and authenticity that reflects their true feelings and thoughts.
For example, a sincere apology is one where the individual truly feels remorse and expresses it without any hidden motives.