assiduous

assiduous \uh-SIJ-oo-uhs\, adjective:

1. Constant in application or attention; devoted; attentive.
2. Performed with constant diligence or attention; unremitting; persistent; as, "assiduous labor."

I can scarcely find time to write you even a Love Letter, Samuel Adams, an assiduous committeeman, wrote his wife in early 1776.
-- Pauline Maier, American Scripture: Making the Declaration of Independence

But he was assiduous in visiting the sick and the poor, however remote their farms and cottages.
-- Jan Morris, "With God where the cuckoos sing", Independent, November 23, 1996

But he was a man who by assiduous reading, through his devotion to literature, had become the quintessential successful gentleman, a man who could hold his own with the most cultivated companions.
-- Milton Gould, quoted in "For Cooke, a Lasting Memorial," by Peter Finn and Richard Justice, Washington Post, April 11, 1997

Assiduous is from Latin assiduus, "constantly sitting near; hence diligent, persistent," from assidere, "to attend to," from ad-, "towards, to" + sedere, "to sit."