Слово "unrelenting". Англо-русский словарь Мюллера

There was the interminable Sunday of his nonage; when his mother, stern of face and unrelenting of heart, would sit all day behind a Bible—bound, like her own construction of it, in the hardest, barest, and straitest boards, with one dinted ornament on the cover like the drag of a chain, and a wrathful sprinkling of red upon the edges of the leaves—as if it, of all books! were a fortification against sweetness of temper, natural affection, and gentle intercourse.

Gnawed within and scorched without, with the infixed, unrelenting fangs of some incurable idea; such an one, could he be found, would seem the very man to dart his iron and lift his lance against the most appalling of all brutes.

Thus every second of Saturday morning was marked by the unrelenting investigation.