Can you suggest some words which can describe a person who is starting to get careless and who slacks, or the very action of becoming careless? A person who was efficiently and elaborately doing their job but with time started to be careless and their performance started to deteriorate.
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"It" cannot be used for people. (With the exception of babies, whose job it is to be careless and slacking.) – RegDwigнt♦ Oct 27 '13 at 13:09
Consider:
complacent :
marked by self-satisfaction especially when accompanied by unawareness of actual dangers or deficiencies
: marked by complacency
- The strong economy has made people complacent.
- We have grown too complacent over the years.
- We can't afford to be complacent about illiteracy.
The person in your question would have become complacent in their job.
When Susan first started, she did her job admirably, but she has recently become complacent and it's starting to show.
Consider coasting
to continue without serious effort, letting one's past efforts carry one along
There is also an idiom phoning it in
Perform an act in a perfunctory, uncommitted fashion, as if it didn't matter.
Please consider:
LACKADAISICAL adjective : feeling or showing a lack of interest or enthusiasm: lacking life, spirit, or zest : languid
from m-w.com
If you are looking for a noun, slacker might fit the bill:
A person who avoids work or effort.