Describes a person or attitude that is earnest, loyal, and without deceit. Somewhat literary or formal in tone.
I was moved by his sincere attitude.
Her earnest approach to work was praised.
Standard kanji spelling for this adjectival noun.
Kana spelling is common in everyday writing, especially when the kanji is less familiar.
Derived from まめ (earnest, diligent) with the adjectival suffix やか. The kanji 忠実 (loyalty + truth) were applied to match the meaning.