sincere and undivided in feeling or spirit; dedicated; not reflecting mixed emotions: He was single-hearted in his patriotism.
Whatever becomes of me, I shall remember always this single-hearted devotion of yours, Margaret, and I shall thank God that I know of it and love you for it.
Edward Boltwood, “The Touchstone,” The Smart Set, May 1910
… one gets what one goes after with single-hearted purpose, but otherwise not.
Anya Seton, The Turquoise, 1946
Single-hearted was first recorded in 1570–80.