The word Slacknes is misspelled against Slackness, a noun meaning "The quality or state of being slack."
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Slackness has double 'ss' in the end mispronounced as 's'
Remember double 'SS' appears in the end of this word which should not be misspelled with single 'S'
Wrong Spelling:  slackne-s 
Correct Spelling:  slackne-ss 
Explore spell check of Slackness to spell without errors.