assiduous:

1. constant; unremitting
2. constant in application or effort; working diligently at a task; persevering; industrious; attentive

how to use in casual conversation:

"he is usually assiduous in his work, i don't know what's going on here"
"being an inherently lazy man, this lifestyle seems too assiduous for me to maintain."
"that girl has one assiduous backside!"

not to be confused with:

assibilate

how it makes me feel:

brilliant! the glory of the presence of the term 'ass' in american slang as a semi-edgy/bad word for ones' gluteus maximus is that any word, any actual legitimate word with 'ass-' as a prefix automatically becomes funny. it's the original 'that's what she said'

i.e.
"hell man, you're assiduous"
"oh yeah? your face is assiduous"

and assibilate? jeez. i could have a freaking field day with that one. the prefix and the suffix are a 12 year old word geeks' wet dream. oh wait. 12 year old word geeks have real wet dreams. you know what i'm talking about though. asserverator? that entry wouldn't stop. it would go on and on for pages. blogspot wouldn't know what to do with it's self. i would win an award for it. "longest and most perverse blog entry that wasn't NSFW".

editorial note:

dictionary.com actually has an entry for the word asshat. MW does not.