Refers to a lack of sincerity or faithfulness in character or actions. Often used in formal or literary contexts.
I was disappointed by his insincere attitude.
Insincerity destroys human relationships.
Compound of 不 (fu-, 'not') + 信実 (shinjitsu, 'sincerity, faithfulness'). The exact historical derivation is uncertain, but the word follows a common pattern of negating a positive quality.