This is a word that I have not come across often, so I had to look it up to make sure I understood its full meaning. I found it this time in “The Abbey Grange” in The Greatest Adventures of Sherlock Holmes.

Assiduously – with great care and perserverance

Usage: The lady had a large, plum colored swelling over one eye “which her maid, a tall, austere woman, was bathing assiduously with vinegar and water.”